--- a/js/src/asmjs/WasmTextToBinary.cpp
+++ b/js/src/asmjs/WasmTextToBinary.cpp
@@ -1725,21 +1725,22 @@ ParseFloatLiteral(WasmParseContext& c, W
             c.ts.generateError(token, c.error);
             return false;
         }
         break;
       case WasmToken::DecNumber: {
         // Call into JS' strtod. Tokenization has already required that the
         // string is well-behaved.
         LifoAlloc::Mark mark = c.lifo.mark();
-        char* buffer = c.lifo.newArray<char>(end - begin + 1);
+        char* buffer = c.lifo.newArray<char>(end - cur + 1);
         if (!buffer)
             return false;
         for (ptrdiff_t i = 0; i < end - cur; ++i)
             buffer[i] = char(cur[i]);
+        buffer[end - cur] = '\0';
         char* strtod_end;
         int err;
         Float d = (Float)js_strtod_harder(c.dtoaState, buffer, &strtod_end, &err);
         if (err != 0 || strtod_end == buffer) {
             c.lifo.release(mark);
             c.ts.generateError(token, c.error);
             return false;
         }