diff --git a/input-keys.c b/input-keys.c
index 76716e0c..096d356f 100644
--- a/input-keys.c
+++ b/input-keys.c
@@ -498,9 +498,8 @@ input_key_vt10x(struct bufferevent *bev, key_code key)
 		return (0);
 	}
 
-	onlykey = key & KEYC_MASK_KEY;
-
 	/* Prevent TAB and RET from being swallowed by C0 remapping logic. */
+	onlykey = key & KEYC_MASK_KEY;
 	if (onlykey == '\r' || onlykey == '\t')
 		key &= ~KEYC_CTRL;
 
@@ -593,7 +592,7 @@ input_key(struct screen *s, struct bufferevent *bev, key_code key)
 
 	/* Is this backtab? */
 	if ((key & KEYC_MASK_KEY) == KEYC_BTAB) {
-		if (s->mode & EXTENDED_KEY_MODES) {
+		if ((s->mode & EXTENDED_KEY_MODES) != 0) {
 			/* When in xterm extended mode, remap into S-Tab. */
 			key = '\011' | (key & ~KEYC_MASK_KEY) | KEYC_SHIFT;
 		} else {
@@ -650,6 +649,13 @@ input_key(struct screen *s, struct bufferevent *bev, key_code key)
 		return (0);
 	}
 
+	/* Ignore internal function key codes. */
+	if ((key >= KEYC_BASE && key < KEYC_BASE_END) ||
+	    (key >= KEYC_USER && key < KEYC_USER_END)) {
+		log_debug("%s: ignoring key 0x%llx", __func__, key);
+		return (0);
+	}
+
 	/*
 	 * No builtin key sequence; construct an extended key sequence
 	 * depending on the client mode.
diff --git a/key-string.c b/key-string.c
index 8258382b..a171b0cb 100644
--- a/key-string.c
+++ b/key-string.c
@@ -410,7 +410,7 @@ key_string_lookup_key(key_code key, int with_flags)
 		s = "MouseMoveBorder";
 		goto append;
 	}
-	if (key >= KEYC_USER && key < KEYC_USER + KEYC_NUSER) {
+	if (key >= KEYC_USER && key < KEYC_USER_END) {
 		snprintf(tmp, sizeof tmp, "User%u", (u_int)(key - KEYC_USER));
 		strlcat(out, tmp, sizeof out);
 		goto out;
diff --git a/tmux.h b/tmux.h
index 77c16b62..197af2c6 100644
--- a/tmux.h
+++ b/tmux.h
@@ -134,6 +134,7 @@ struct winlink;
  */
 #define KEYC_BASE            0x0000000010e000ULL
 #define KEYC_USER            0x0000000010f000ULL
+#define KEYC_USER_END	     (KEYC_USER + KEYC_NUSER)
 
 /* Key modifier bits. */
 #define KEYC_META            0x00100000000000ULL
@@ -168,7 +169,7 @@ struct winlink;
 	 (((key) & KEYC_MASK_KEY) < KEYC_BASE || \
 	  ((key) & KEYC_MASK_KEY) >= KEYC_BASE_END) && \
 	 (((key) & KEYC_MASK_KEY) < KEYC_USER || \
-	  ((key) & KEYC_MASK_KEY) >= KEYC_USER + KEYC_NUSER))
+	  ((key) & KEYC_MASK_KEY) >= KEYC_USER_END))
 
 /* Multiple click timeout. */
 #define KEYC_CLICK_TIMEOUT 300