diff --git a/libguile/goops.c b/libguile/goops.c
index 8da4a0772..5708059e5 100644
--- a/libguile/goops.c
+++ b/libguile/goops.c
@@ -2377,6 +2377,7 @@ create_standard_classes (void)
 	       scm_class_class,
 	       scm_list_2 (scm_class_input_port, scm_class_output_port),
 	       SCM_EOL);
+  scm_classes_initialized = 1;
 }
 
 /**********************************************************************
diff --git a/libguile/objects.c b/libguile/objects.c
index f81014738..ad1501e95 100644
--- a/libguile/objects.c
+++ b/libguile/objects.c
@@ -78,6 +78,8 @@ SCM scm_class_unknown;
 SCM *scm_port_class = 0;
 SCM *scm_smob_class = 0;
 
+int scm_classes_initialized = 0;
+
 SCM scm_no_applicable_method;
 
 /* This function is used for efficient type dispatch.  */
@@ -86,6 +88,9 @@ SCM_DEFINE (scm_class_of, "class-of", 1, 0, 0,
 	    "Return the class of @var{x}.")
 #define FUNC_NAME s_scm_class_of
 {
+  if (!scm_classes_initialized)
+    scm_misc_error (FUNC_NAME, "GOOPS not loaded yet.", SCM_EOL);
+
   switch (SCM_ITAG3 (x))
     {
     case scm_tc3_int_1:
diff --git a/libguile/objects.h b/libguile/objects.h
index 649d3fb1e..54c40a913 100644
--- a/libguile/objects.h
+++ b/libguile/objects.h
@@ -207,6 +207,7 @@ extern SCM scm_class_real, scm_class_complex, scm_class_integer;
 extern SCM scm_class_unknown;
 extern SCM *scm_port_class;
 extern SCM *scm_smob_class;
+extern int scm_classes_initialized;
 
 extern SCM scm_no_applicable_method;