diff --git a/README.md b/README.md
index 6d15897..eea37ca 100644
--- a/README.md
+++ b/README.md
@@ -295,13 +295,13 @@ otherwise return `null`.
 
 #### & *expression*
 
-Try to match the expression. If the match succeeds, just return an empty string
-and do not advance the parser position, otherwise consider the match failed.
+Try to match the expression. If the match succeeds, just return `undefined` and
+do not advance the parser position, otherwise consider the match failed.
 
 #### ! *expression*
 
-Try to match the expression. If the match does not succeed, just return an empty
-string and do not advance the parser position, otherwise consider the match
+Try to match the expression. If the match does not succeed, just return
+`undefined` and do not advance the parser position, otherwise consider the match
 failed.
 
 #### & { *predicate* }
@@ -310,7 +310,7 @@ The predicate is a piece of JavaScript code that is executed as if it was inside
 a function. It gets the match results of labeled expressions in preceding
 expression as its arguments. It should return some JavaScript value using the
 `return` statement. If the returned value evaluates to `true` in boolean
-context, just return an empty string and do not advance the parser position;
+context, just return `undefined` and do not advance the parser position;
 otherwise consider the match failed.
 
 The code inside the predicate can access all variables and functions defined in
@@ -332,7 +332,7 @@ The predicate is a piece of JavaScript code that is executed as if it was inside
 a function. It gets the match results of labeled expressions in preceding
 expression as its arguments. It should return some JavaScript value using the
 `return` statement. If the returned value evaluates to `false` in boolean
-context, just return an empty string and do not advance the parser position;
+context, just return `undefined` and do not advance the parser position;
 otherwise consider the match failed.
 
 The code inside the predicate can access all variables and functions defined in
diff --git a/lib/compiler/passes/generate-bytecode.js b/lib/compiler/passes/generate-bytecode.js
index 9709f9e..9f13c7b 100644
--- a/lib/compiler/passes/generate-bytecode.js
+++ b/lib/compiler/passes/generate-bytecode.js
@@ -213,8 +213,8 @@ module.exports = function(ast, options) {
   }
 
   function buildSimplePredicate(expression, negative, context) {
-    var emptyStringIndex = addConst('""'),
-        failedIndex      = addConst('peg$FAILED');
+    var undefinedIndex = addConst('void 0'),
+        failedIndex    = addConst('peg$FAILED');
 
     return buildSequence(
       [op.PUSH_CURR_POS],
@@ -230,7 +230,7 @@ module.exports = function(ast, options) {
         buildSequence(
           [op.POP],
           [negative ? op.POP : op.POP_CURR_POS],
-          [op.PUSH, emptyStringIndex]
+          [op.PUSH, undefinedIndex]
         ),
         buildSequence(
           [op.POP],
@@ -242,9 +242,9 @@ module.exports = function(ast, options) {
   }
 
   function buildSemanticPredicate(code, negative, context) {
-    var functionIndex    = addFunctionConst(utils.keys(context.env), code),
-        emptyStringIndex = addConst('""'),
-        failedIndex      = addConst('peg$FAILED');
+    var functionIndex  = addFunctionConst(utils.keys(context.env), code),
+        undefinedIndex = addConst('void 0'),
+        failedIndex    = addConst('peg$FAILED');
 
     return buildSequence(
       [op.REPORT_CURR_POS],
@@ -253,11 +253,11 @@ module.exports = function(ast, options) {
         [op.IF],
         buildSequence(
           [op.POP],
-          [op.PUSH, negative ? failedIndex : emptyStringIndex]
+          [op.PUSH, negative ? failedIndex : undefinedIndex]
         ),
         buildSequence(
           [op.POP],
-          [op.PUSH, negative ? emptyStringIndex : failedIndex]
+          [op.PUSH, negative ? undefinedIndex : failedIndex]
         )
       )
     );
diff --git a/lib/parser.js b/lib/parser.js
index e10921e..65ee165 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -197,7 +197,7 @@ module.exports = (function() {
                 expression: expression
               };
             },
-        peg$c18 = "",
+        peg$c18 = void 0,
         peg$c19 = function(name) {
               return {
                 type: "rule_ref",
diff --git a/spec/compiler/passes/generate-bytecode.spec.js b/spec/compiler/passes/generate-bytecode.spec.js
index d9a1e4e..84ed6b6 100644
--- a/spec/compiler/passes/generate-bytecode.spec.js
+++ b/spec/compiler/passes/generate-bytecode.spec.js
@@ -272,7 +272,7 @@ describe("compiler pass |generateBytecode|", function() {
 
     it("defines correct constants", function() {
       expect(pass).toChangeAST(grammar, constsDetails([
-        '""',
+        'void 0',
         'peg$FAILED',
         '"a"',
         '{ type: "literal", value: "a", description: "\\"a\\"" }'
@@ -301,7 +301,7 @@ describe("compiler pass |generateBytecode|", function() {
 
     it("defines correct constants", function() {
       expect(pass).toChangeAST(grammar, constsDetails([
-        '""',
+        'void 0',
         'peg$FAILED',
         '"a"',
         '{ type: "literal", value: "a", description: "\\"a\\"" }'
@@ -328,7 +328,7 @@ describe("compiler pass |generateBytecode|", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST(
           grammar,
-          constsDetails(['function() { code }', '""', 'peg$FAILED'])
+          constsDetails(['function() { code }', 'void 0', 'peg$FAILED'])
         );
       });
     });
@@ -380,7 +380,7 @@ describe("compiler pass |generateBytecode|", function() {
           '"c"',
           '{ type: "literal", value: "c", description: "\\"c\\"" }',
           'function(a, b, c) { code }',
-          '""'
+          'void 0'
         ]));
       });
     });
@@ -405,7 +405,7 @@ describe("compiler pass |generateBytecode|", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST(
           grammar,
-          constsDetails(['function() { code }', '""', 'peg$FAILED'])
+          constsDetails(['function() { code }', 'void 0', 'peg$FAILED'])
         );
       });
     });
@@ -457,7 +457,7 @@ describe("compiler pass |generateBytecode|", function() {
           '"c"',
           '{ type: "literal", value: "c", description: "\\"c\\"" }',
           'function(a, b, c) { code }',
-          '""'
+          'void 0'
         ]));
       });
     });
diff --git a/spec/generated-parser.spec.js b/spec/generated-parser.spec.js
index c0fcff1..8abbb73 100644
--- a/spec/generated-parser.spec.js
+++ b/spec/generated-parser.spec.js
@@ -397,14 +397,14 @@ describe("generated parser", function() {
       it("matches correctly", function() {
         var parser = PEG.buildParser('start = &"a" "a"', options);
 
-        expect(parser).toParse("a", ["", "a"]);
+        expect(parser).toParse("a", [undefined, "a"]);
         expect(parser).toFailToParse("b");
       });
 
       it("does not advance position on success", function() {
         var parser = PEG.buildParser('start = &"a" "a"', options);
 
-        expect(parser).toParse("a", ["", "a"]);
+        expect(parser).toParse("a", [undefined, "a"]);
       });
 
       it("does not influence expected strings on failure", function() {
@@ -418,7 +418,7 @@ describe("generated parser", function() {
       it("matches correctly", function() {
         var parser = PEG.buildParser('start = !"a" "b"', options);
 
-        expect(parser).toParse("b", ["", "b"]);
+        expect(parser).toParse("b", [undefined, "b"]);
         expect(parser).toFailToParse("a");
       });
 
@@ -439,7 +439,7 @@ describe("generated parser", function() {
       it("causes successful match by returning |true|", function() {
         var parser = PEG.buildParser('start = &{ return true; }', options);
 
-        expect(parser).toParse("", "");
+        expect(parser).toParse("", undefined);
       });
 
       it("causes match failure by returning |false|", function() {
@@ -454,7 +454,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |text| function", function() {
@@ -463,7 +463,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |offset| function to get the current parse position", function() {
@@ -472,7 +472,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |line| and |column| functions to get the current line and column", function() {
@@ -504,7 +504,7 @@ describe("generated parser", function() {
               'start = "a" &{ return v === 42; }'
             ].join("\n"), options);
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use functions defined in the initializer", function() {
@@ -513,7 +513,7 @@ describe("generated parser", function() {
               'start = "a" &{ return f() === 42; }'
             ].join("\n"), options);
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use options passed to the parser", function() {
@@ -530,7 +530,7 @@ describe("generated parser", function() {
       it("causes successful match by returning |false|", function() {
         var parser = PEG.buildParser('start = !{ return false; }', options);
 
-        expect(parser).toParse("", "");
+        expect(parser).toParse("", undefined);
       });
 
       it("causes match failure by returning |true|", function() {
@@ -545,7 +545,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |text| function", function() {
@@ -554,7 +554,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |offset| function to get the current parse position", function() {
@@ -563,7 +563,7 @@ describe("generated parser", function() {
               options
             );
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use the |line| and |column| functions to get the current line and column", function() {
@@ -595,7 +595,7 @@ describe("generated parser", function() {
               'start = "a" !{ return v !== 42; }'
             ].join("\n"), options);
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use functions defined in the initializer", function() {
@@ -604,7 +604,7 @@ describe("generated parser", function() {
               'start = "a" !{ return f() !== 42; }'
             ].join("\n"), options);
 
-        expect(parser).toParse("a", ["a", ""]);
+        expect(parser).toParse("a", ["a", undefined]);
       });
 
       it("can use options passed to the parser", function() {