diff --git a/pykwasm/src/pykwasm/kdist/wasm-semantics/binary-parsing/instr.md b/pykwasm/src/pykwasm/kdist/wasm-semantics/binary-parsing/instr.md
index c52b0f71c..ce6359772 100644
--- a/pykwasm/src/pykwasm/kdist/wasm-semantics/binary-parsing/instr.md
+++ b/pykwasm/src/pykwasm/kdist/wasm-semantics/binary-parsing/instr.md
@@ -509,9 +509,19 @@ module BINARY-PARSER-INSTR
   rule #parseInstrx62s1(memArgResult(MemArg2:MemArg, BWI:BytesWithIndex)) => instrResult(#store(i64, store32, getMemArgOffset(MemArg2)), BWI)
   rule #parseInstrx62s1(E:ParseError) => E
   syntax InstrResult ::= parseInstrx63(BytesWithIndex)  [function, total]
-  rule parseInstrx63(BWI:BytesWithIndex) => instrResult(memory.size, BWI)
+  syntax InstrResult ::= #parseInstrx63s1
+                            ( BytesWithIndexOrError
+                            )  [function, total]
+  rule parseInstrx63(BWI) => #parseInstrx63s1(parseConstant(BWI, b"\x00"))
+  rule #parseInstrx63s1(BWI:BytesWithIndex) => instrResult(memory.size, BWI)
+  rule #parseInstrx63s1(E:ParseError) => E
   syntax InstrResult ::= parseInstrx64(BytesWithIndex)  [function, total]
-  rule parseInstrx64(BWI:BytesWithIndex) => instrResult(memory.grow, BWI)
+  syntax InstrResult ::= #parseInstrx64s1
+                            ( BytesWithIndexOrError
+                            )  [function, total]
+  rule parseInstrx64(BWI) => #parseInstrx64s1(parseConstant(BWI, b"\x00"))
+  rule #parseInstrx64s1(BWI:BytesWithIndex) => instrResult(memory.grow, BWI)
+  rule #parseInstrx64s1(E:ParseError) => E
   syntax InstrResult ::= parseInstrx65(BytesWithIndex)  [function, total]
   syntax InstrResult ::= #parseInstrx65s1
                             ( IntResult
diff --git a/pykwasm/src/pykwasm/scripts/binary-parser-gen.py b/pykwasm/src/pykwasm/scripts/binary-parser-gen.py
index 71b24832c..7e5fc92c5 100644
--- a/pykwasm/src/pykwasm/scripts/binary-parser-gen.py
+++ b/pykwasm/src/pykwasm/scripts/binary-parser-gen.py
@@ -404,8 +404,8 @@ def with_prefix(self, p: bytes) -> 'InstrConfig':
     InstrConfig('I64_STORE_8', b'\x3C', [constructor_arg('i64'), constructor_arg('store8'), 'MemArg', ], '#store'),
     InstrConfig('I64_STORE_16', b'\x3D', [constructor_arg('i64'), constructor_arg('store16'), 'MemArg', ], '#store'),
     InstrConfig('I64_STORE_32', b'\x3E', [constructor_arg('i64'), constructor_arg('store32'), 'MemArg', ], '#store'),
-    InstrConfig('MEM_SIZE', b'\x3F', [], 'memory.size'),
-    InstrConfig('MEM_GROW', b'\x40', [], 'memory.grow'),
+    InstrConfig('MEM_SIZE', b'\x3F', [parse_constant(b'\x00')], 'memory.size'),
+    InstrConfig('MEM_GROW', b'\x40', [parse_constant(b'\x00')], 'memory.grow'),
     InstrConfig('MEM_INIT', b'\xFC\x08', ['UnsignedInt', parse_constant(b'\x00'), ], not_implemented()),
     InstrConfig('DATA_DROP', b'\xFC\x09', ['UnsignedInt', ], not_implemented()),
     InstrConfig('MEM_COPY', b'\xFC\x0A', [parse_constant(b'\x00'), parse_constant(b'\x00'), ], 'memory.copy'),