diff --git a/libsrc/memio.c b/libsrc/memio.c
index d16b1c3c6..cb2a8e627 100644
--- a/libsrc/memio.c
+++ b/libsrc/memio.c
@@ -446,7 +446,7 @@ memio_pad_length(ncio* nciop, off_t length)
     if(nciop == NULL || nciop->pvt == NULL) return NC_EINVAL;
     memio = (NCMEMIO*)nciop->pvt;
 
-    if(fIsSet(nciop->ioflags,NC_WRITE))
+    if(!fIsSet(nciop->ioflags,NC_WRITE))
         return EPERM; /* attempt to write readonly file*/
     if(memio->locked)
 	return NC_EINMEMORY;