diff --git a/src/posix.c b/src/posix.c
index e026480e..45dbf0aa 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -112,8 +112,9 @@ SCM
 read_char (SCM port)            /*:((arity . n)) */
 {
   int fd = __stdin;
-  if (TYPE (port) == TPAIR && TYPE (car (port)) == TNUMBER)
-    __stdin = VALUE (CAR (port));
+  if (TYPE (port) == TPAIR)
+    if (TYPE (CAR (port)) == TNUMBER)
+      __stdin = VALUE (CAR (port));
   SCM c = make_char (readchar ());
   __stdin = fd;
   return c;