diff --git a/integration/simple-long/simple.ts b/integration/simple-long/simple.ts
index edbc8b312..579f37c6c 100644
--- a/integration/simple-long/simple.ts
+++ b/integration/simple-long/simple.ts
@@ -226,7 +226,7 @@ export const SimpleWithMap = {
     message.longLookup = {};
     if (object.longLookup !== undefined && object.longLookup !== null) {
       Object.entries(object.longLookup).forEach(([key, value]) => {
-        message.longLookup[key] = Long.fromValue(value as string);
+        message.longLookup[key] = Long.fromValue(value as Long | string);
       });
     }
     return message;
diff --git a/src/main.ts b/src/main.ts
index 41662fca5..77d3320c7 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1047,7 +1047,7 @@ function generateFromJson(ctx: Context, fullName: string, messageDesc: Descripto
                 return code`${utils.bytesFromBase64}(${from} as string)`;
               }
             } else if (isLong(valueType) && options.forceLong === LongOption.LONG) {
-              return code`Long.fromValue(${from} as string)`;
+              return code`Long.fromValue(${from} as Long | string)`;
             } else if (isEnum(valueType)) {
               return code`${from} as number`;
             } else {