diff --git a/src/cmp/cmpct.lsp b/src/cmp/cmpct.lsp
index 5adb949a4..d5c65d5af 100644
--- a/src/cmp/cmpct.lsp
+++ b/src/cmp/cmpct.lsp
@@ -32,21 +32,27 @@
     (make-c1form* 'LOCATION :type 'CHARACTER
 		  :args (list 'CHARACTER-VALUE (char-code val))))
    ((typep val 'DOUBLE-FLOAT)
+    (when (and (ext:float-nan-p val) (not only-small-values))
+      (cmperr "Cannot externalize value ~A" val))
     (make-c1form* 'LOCATION :type 'DOUBLE-FLOAT
 		  :args (list 'DOUBLE-FLOAT-VALUE val (add-object val))))
    ((typep val 'SINGLE-FLOAT)
+    (when (and (ext:float-nan-p val) (not only-small-values))
+      (cmperr "Cannot externalize value ~A" val))
     (make-c1form* 'LOCATION :type 'SINGLE-FLOAT
 		  :args (list 'SINGLE-FLOAT-VALUE val (add-object val))))
    ((typep val 'LONG-FLOAT)
+    (when (and (ext:float-nan-p val) (not only-small-values))
+      (cmperr "Cannot externalize value ~A" val))
     (make-c1form* 'LOCATION :type 'LONG-FLOAT
 		  :args (list 'LONG-FLOAT-VALUE val (add-object val))))
    #+sse2
    ((typep val 'EXT:SSE-PACK)
     (c1constant-value/sse val))
+   (only-small-values nil)
    (always
     (make-c1form* 'LOCATION :type (object-type val)
 		  :args (add-object val)))
-   (only-small-values nil)
    (t nil)))
 
 #+sse2