diff --git a/src/wren_core.c b/src/wren_core.c
index 673fa014..174fcd51 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -436,35 +436,30 @@ DEF_NATIVE(num_divide)
 DEF_NATIVE(num_mod)
 {
   if (!validateNum(vm, args, 1, "Right operand")) return PRIM_ERROR;
-  if (!IS_NUM(args[1])) RETURN_NULL;
   RETURN_NUM(fmod(AS_NUM(args[0]), AS_NUM(args[1])));
 }
 
 DEF_NATIVE(num_lt)
 {
-  // TODO: Error on wrong argument type.
-  if (!IS_NUM(args[1])) RETURN_NULL;
+  if (!validateNum(vm, args, 1, "Right operand")) return PRIM_ERROR;
   RETURN_BOOL(AS_NUM(args[0]) < AS_NUM(args[1]));
 }
 
 DEF_NATIVE(num_gt)
 {
-  // TODO: Error on wrong argument type.
-  if (!IS_NUM(args[1])) RETURN_NULL;
+  if (!validateNum(vm, args, 1, "Right operand")) return PRIM_ERROR;
   RETURN_BOOL(AS_NUM(args[0]) > AS_NUM(args[1]));
 }
 
 DEF_NATIVE(num_lte)
 {
-  // TODO: Error on wrong argument type.
-  if (!IS_NUM(args[1])) RETURN_NULL;
+  if (!validateNum(vm, args, 1, "Right operand")) return PRIM_ERROR;
   RETURN_BOOL(AS_NUM(args[0]) <= AS_NUM(args[1]));
 }
 
 DEF_NATIVE(num_gte)
 {
-  // TODO: Error on wrong argument type.
-  if (!IS_NUM(args[1])) RETURN_NULL;
+  if (!validateNum(vm, args, 1, "Right operand")) return PRIM_ERROR;
   RETURN_BOOL(AS_NUM(args[0]) >= AS_NUM(args[1]));
 }
 
diff --git a/test/number/greater_than_equal_operand_not_num.wren b/test/number/greater_than_equal_operand_not_num.wren
new file mode 100644
index 00000000..b4c452c9
--- /dev/null
+++ b/test/number/greater_than_equal_operand_not_num.wren
@@ -0,0 +1 @@
+1 >= false // expect runtime error: Right operand must be a number.
diff --git a/test/number/greater_than_operand_not_num.wren b/test/number/greater_than_operand_not_num.wren
new file mode 100644
index 00000000..839e7f62
--- /dev/null
+++ b/test/number/greater_than_operand_not_num.wren
@@ -0,0 +1 @@
+1 > false // expect runtime error: Right operand must be a number.
diff --git a/test/number/less_than_equal_operand_not_num.wren b/test/number/less_than_equal_operand_not_num.wren
new file mode 100644
index 00000000..0a126819
--- /dev/null
+++ b/test/number/less_than_equal_operand_not_num.wren
@@ -0,0 +1 @@
+1 <= false // expect runtime error: Right operand must be a number.
diff --git a/test/number/less_than_operand_not_num.wren b/test/number/less_than_operand_not_num.wren
new file mode 100644
index 00000000..33fd4aac
--- /dev/null
+++ b/test/number/less_than_operand_not_num.wren
@@ -0,0 +1 @@
+1 < false // expect runtime error: Right operand must be a number.