diff --git a/library/pkwrite.c b/library/pkwrite.c
index 51d0c56..8d1da2f 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -186,7 +186,9 @@
     const char *oid;
 
     PK_VALIDATE_RET( key != NULL );
-    PK_VALIDATE_RET( buf != NULL || size == 0 );
+    if( size == 0 )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+    PK_VALIDATE_RET( buf != NULL );
 
     c = buf + size;
 
@@ -236,7 +238,9 @@
     size_t len = 0;
 
     PK_VALIDATE_RET( key != NULL );
-    PK_VALIDATE_RET( buf != NULL || size == 0 );
+    if( size == 0 )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+    PK_VALIDATE_RET( buf != NULL );
 
     c = buf + size;