diff --git a/src/command/add.js b/src/command/add.js
index ba77cbf..e6752c4 100644
--- a/src/command/add.js
+++ b/src/command/add.js
@@ -33,7 +33,7 @@ Juicescript.command_define({
 			let data_type = runner.data_type(add);
 			if(data_type !== Juicescript.data_type.NUM && data_type !== Juicescript.data_type.STR){
 				// ignore with warning
-				runner.warning_argument(q, "invalid data type " + Juicescript.data_type.name(data_type));
+				runner.warning_argument(q, "expected data type NUM or STR, but got " + Juicescript.data_type.name(data_type));
 				continue;
 			}
 			
diff --git a/src/command/div.js b/src/command/div.js
index 0b69f28..1692228 100644
--- a/src/command/div.js
+++ b/src/command/div.js
@@ -33,7 +33,7 @@ Juicescript.command_define({
 			let data_type = runner.data_type(devide);
 			if(data_type !== Juicescript.data_type.NUM){
 				// ignore with warning
-				runner.warning_argument(q, "invalid data type " + Juicescript.data_type.name(data_type));
+				runner.warning_argument(q, "expected data type NUM, but got " + Juicescript.data_type.name(data_type));
 				continue;
 			}
 			
diff --git a/src/command/jmp.js b/src/command/jmp.js
index 2a12ab4..6341f53 100644
--- a/src/command/jmp.js
+++ b/src/command/jmp.js
@@ -16,6 +16,14 @@ Juicescript.command_define({
 		// flag name
 		let flag = runner.argument_value(1);
 		
+		// validate data type
+		let data_type = runner.data_type(flag);
+		if(data_type !== Juicescript.data_type.STR){
+			// ignore with warning
+			runner.warning_argument(1, "expected data type STR, but got " + Juicescript.data_type.name(data_type));
+			return;
+		}
+		
 		// load this scope's flag list
 		let flag_list = runner.scope_tree.flag;
 		
diff --git a/src/command/mul.js b/src/command/mul.js
index e11f184..90767ad 100644
--- a/src/command/mul.js
+++ b/src/command/mul.js
@@ -35,7 +35,7 @@ Juicescript.command_define({
 			let factor_data_type = runner.data_type(factor);
 			if(factor_data_type !== Juicescript.data_type.NUM){
 				// ignore with warning
-				runner.warning_argument(q, "invalid data type " + Juicescript.data_type.name(factor_data_type));
+				runner.warning_argument(q, "expected data type NUM, but got " + Juicescript.data_type.name(factor_data_type));
 				continue;
 			}
 			
diff --git a/src/command/sub.js b/src/command/sub.js
index 0d69c40..c6b12d6 100644
--- a/src/command/sub.js
+++ b/src/command/sub.js
@@ -33,7 +33,7 @@ Juicescript.command_define({
 			let data_type = runner.data_type(subtract);
 			if(data_type !== Juicescript.data_type.NUM){
 				// ignore with warning
-				runner.warning_argument(q, "invalid data type " + Juicescript.data_type.name(data_type));
+				runner.warning_argument(q, "expected data type NUM, but got " + Juicescript.data_type.name(data_type));
 				continue;
 			}