diff --git a/src/libsysprof-analyze/sysprof-strings.c b/src/libsysprof-analyze/sysprof-strings.c
index 319d4219..a7c13f4e 100644
--- a/src/libsysprof-analyze/sysprof-strings.c
+++ b/src/libsysprof-analyze/sysprof-strings.c
@@ -70,6 +70,9 @@ sysprof_strings_get (SysprofStrings *self,
 {
   GRefString *ret;
 
+  if (string == NULL)
+    return NULL;
+
   g_mutex_lock (&self->mutex);
   if (!(ret = g_hash_table_lookup (self->hashtable, string)))
     {
@@ -88,6 +91,9 @@ sysprof_strings_lookup (SysprofStrings *self,
 {
   GRefString *ret;
 
+  if (string == NULL)
+    return NULL;
+
   g_mutex_lock (&self->mutex);
   ret = g_hash_table_lookup (self->hashtable, string);
   g_mutex_unlock (&self->mutex);