diff --git a/src/parse.y b/src/parse.y
index 252b7390e2..258c3d9c86 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -247,6 +247,15 @@ static bool expr_is_table_type_name(const Expr* expr)
 	return false;
 	}
 
+static void check_loop_var(const IDPtr& var)
+	{
+	if ( var->IsGlobal() )
+ 		var->Error("global variable used in 'for' loop");
+
+ 	if ( var->IsConst() )
+ 		var->Error("constant used in 'for' loop");
+	}
+
 static void build_global(ID* id, Type* t, InitClass ic, Expr* e,
                          std::vector<AttrPtr>* attrs, DeclType dt)
 	{
@@ -1908,11 +1917,7 @@ for_head:
 			auto loop_var = lookup_ID($3, current_module.c_str());
 
 			if ( loop_var )
-				{
-				if ( loop_var->IsGlobal() )
-					loop_var->Error("global variable used in for loop");
-				}
-
+				check_loop_var(loop_var);
 			else
 				{
 				loop_var = install_ID($3, current_module.c_str(),
@@ -1942,18 +1947,12 @@ for_head:
 
 			// Validate previous definitions as needed.
 			if ( key_var )
-				{
-				if ( key_var->IsGlobal() )
-					key_var->Error("global variable used in for loop");
-				}
+				check_loop_var(key_var);
 			else
 				key_var = install_ID($3, module, false, false);
 
 			if ( val_var )
-				{
-				if ( val_var->IsGlobal() )
-					val_var->Error("global variable used in for loop");
-				}
+				check_loop_var(val_var);
 			else
 				val_var = install_ID($5, module, false, false);
 
@@ -1972,10 +1971,7 @@ for_head:
 			auto val_var = lookup_ID($7, module);
 
 			if ( val_var )
-				{
-				if ( val_var->IsGlobal() )
-					val_var->Error("global variable used in for loop");
-				}
+				check_loop_var(val_var);
 			else
 				val_var = install_ID($7, module, false, false);
 
@@ -2004,6 +2000,10 @@ local_id:
 				{
 				if ( $$->IsGlobal() )
 					$$->Error("already a global identifier");
+
+				if ( $$->IsConst() )
+ 					$$->Error("already a const identifier");
+
 				delete [] $1;
 				}
 
diff --git a/testing/btest/Baseline/language.for-var-check/out b/testing/btest/Baseline/language.for-var-check/out
new file mode 100644
index 0000000000..4d222556e0
--- /dev/null
+++ b/testing/btest/Baseline/language.for-var-check/out
@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/for-var-check.zeek, line 4: global variable used in 'for' loop (x)
+error in error and count: type clash in iteration (error and count)
+error in <...>/for-var-check.zeek, line 8: constant used in 'for' loop (y)
+error in error and count: type clash in iteration (error and count)
diff --git a/testing/btest/language/for-var-check.zeek b/testing/btest/language/for-var-check.zeek
new file mode 100644
index 0000000000..d33cb2f6ac
--- /dev/null
+++ b/testing/btest/language/for-var-check.zeek
@@ -0,0 +1,13 @@
+# @TEST-EXEC-FAIL: zeek -b %INPUT >out 2>&1
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff out
+
+global x: count = 0;
+
+event zeek_init()
+	{
+	const y: count = 0;
+
+	for ( x in set(1, 2, 3) ) {}
+	for ( y in set(1, 2, 3) ) {}
+	}
+