diff --git a/mono/metadata/class-internals.h b/mono/metadata/class-internals.h
index 39061bd142a1..ef8127e14a4a 100644
--- a/mono/metadata/class-internals.h
+++ b/mono/metadata/class-internals.h
@@ -492,7 +492,7 @@ struct MonoVTable {
  */
 struct _MonoGenericInst {
 #ifndef MONO_SMALL_CONFIG
-	guint id;			/* unique ID for debugging */
+	gint32 id;			/* unique ID for debugging */
 #endif
 	guint type_argc    : 22;	/* number of type arguments */
 	guint is_open      :  1;	/* if this is an open type */
diff --git a/mono/metadata/metadata.c b/mono/metadata/metadata.c
index 651538c3c5cb..aea1343df99c 100644
--- a/mono/metadata/metadata.c
+++ b/mono/metadata/metadata.c
@@ -1522,7 +1522,7 @@ builtin_types[] = {
 #define NBUILTIN_TYPES() (sizeof (builtin_types) / sizeof (builtin_types [0]))
 
 static GHashTable *type_cache = NULL;
-static int next_generic_inst_id = 0;
+static gint32 next_generic_inst_id = 0;
 
 /* Protected by image_sets_mutex */
 static MonoImageSet *mscorlib_image_set;
@@ -3160,7 +3160,7 @@ mono_metadata_get_canonical_generic_inst (MonoGenericInst *candidate)
 		int size = MONO_SIZEOF_GENERIC_INST + type_argc * sizeof (MonoType *);
 		ginst = (MonoGenericInst *)mono_image_set_alloc0 (set, size);
 #ifndef MONO_SMALL_CONFIG
-		ginst->id = ++next_generic_inst_id;
+		ginst->id = InterlockedIncrement (&next_generic_inst_id);
 #endif
 		ginst->is_open = is_open;
 		ginst->type_argc = type_argc;