diff --git a/assembly/encoder.ts b/assembly/encoder.ts
index c1e5d67..ec64a92 100644
--- a/assembly/encoder.ts
+++ b/assembly/encoder.ts
@@ -79,7 +79,7 @@ export class JSONEncoder {
       this._isFirstKey[this._isFirstKey.length - 1] = 0;
     }
     if (str != null && (<string>str).length > 0) {
-      this.writeString(str!);
+      this.writeString(str);
       this.write(":");
     }
   }