diff --git a/image-sixel.c b/image-sixel.c
index 1d814015c..9b0b412ae 100644
--- a/image-sixel.c
+++ b/image-sixel.c
@@ -823,7 +823,7 @@ static void
 sixel_print_add(char **buf, size_t *len, size_t *used, const char *s,
     size_t slen)
 {
-	while (*used + slen >= *len + 1) {
+	while (*used + slen + 1 > *len) {
 		*buf = xreallocarray(*buf, 2, *len);
 		(*len) *= 2;
 	}
diff --git a/regress/image-sixel-buffer.c b/regress/image-sixel-buffer.c
new file mode 100644
index 000000000..d6869bc55
--- /dev/null
+++ b/regress/image-sixel-buffer.c
@@ -0,0 +1,72 @@
+/* Exercise SIXEL output lengths at the encoder's allocation boundary. */
+
+#undef NDEBUG
+#include <assert.h>
+
+#include "../tmux.h"
+
+#ifdef ENABLE_IMAGES
+#include "../image-sixel.c"
+#include "../xmalloc.c"
+
+/* Abort if an allocation or formatting check fails. */
+void
+fatal(__unused const char *fmt, ...)
+{
+	abort();
+}
+
+/* Abort if an allocation or formatting check fails. */
+void
+fatalx(__unused const char *fmt, ...)
+{
+	abort();
+}
+
+/* Encode lengths below, at and above the initial 8192-byte allocation. */
+int
+main(void)
+{
+	struct sixel_image	 image = { 0 };
+	u_int			 colour, width, x;
+	size_t			 size;
+	char			*data;
+
+	colour = (2u << 25) | (100u << 16) | (100u << 8) | 100u;
+	image.sy = 2;
+	image.ncolours = 1;
+	image.used_colours = 1;
+	image.colours = &colour;
+	image.lines = xcalloc(2, sizeof *image.lines);
+
+	/* Headers and controls add 26 bytes to these uncompressed rasters. */
+	for (width = 8165; width <= 8167; width++) {
+		image.sx = width;
+		image.lines[0].sx = width;
+		image.lines[1].sx = width;
+		image.lines[0].pixels = xcalloc(width, sizeof(uint16_t));
+		image.lines[1].pixels = xcalloc(width, sizeof(uint16_t));
+
+		/* Alternate row bits so adjacent columns cannot be compressed. */
+		for (x = 0; x < width; x++)
+			image.lines[x % 2].pixels[x] = 1;
+		data = sixel_print(&image, NULL, &size);
+		assert(data != NULL);
+		assert(size == width + 26);
+		assert(data[size] == '\0');
+		assert(strlen(data) == size);
+		free(data);
+		free(image.lines[0].pixels);
+		free(image.lines[1].pixels);
+	}
+	free(image.lines);
+	return (0);
+}
+#else
+/* Skip the encoder check when image support is disabled. */
+int
+main(void)
+{
+	return (0);
+}
+#endif
diff --git a/regress/image-sixel-buffer.mk b/regress/image-sixel-buffer.mk
new file mode 100644
index 000000000..e76d1457c
--- /dev/null
+++ b/regress/image-sixel-buffer.mk
@@ -0,0 +1,15 @@
+# Use the configured compiler and flags for the standalone encoder check.
+
+.PHONY: sixel-buffer-probe sixel-buffer-test
+
+sixel-buffer-probe:
+	$(CC) $(CFLAGS) $(LDFLAGS) -fsanitize=address \
+	    -o "$(SIXEL_TEST_DIR)/probe" "$(SIXEL_TEST_DIR)/probe.c"
+
+sixel-buffer-test:
+	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
+	    $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) -UNDEBUG \
+	    -ffunction-sections -fdata-sections -fsanitize=address \
+	    -fno-omit-frame-pointer $(SIXEL_TEST_LDFLAGS) $(LDFLAGS) \
+	    -o "$(SIXEL_TEST_DIR)/test" "$(srcdir)/regress/image-sixel-buffer.c" \
+	    "$(srcdir)/compat/reallocarray.c"
diff --git a/regress/image-sixel-buffer.sh b/regress/image-sixel-buffer.sh
new file mode 100644
index 000000000..142a5715f
--- /dev/null
+++ b/regress/image-sixel-buffer.sh
@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# Catch a trailing NUL written beyond an exactly full SIXEL output buffer.
+
+PATH=/bin:/usr/bin
+export PATH
+
+SIXEL_TEST_DIR=$(mktemp -d) || exit 1
+trap 'rm -rf "$SIXEL_TEST_DIR"' 0 1 15
+export SIXEL_TEST_DIR
+ASAN_OPTIONS=detect_leaks=0
+export ASAN_OPTIONS
+
+case $(uname -s) in
+Darwin) SIXEL_TEST_LDFLAGS=-Wl,-dead_strip ;;
+*) SIXEL_TEST_LDFLAGS=-Wl,--gc-sections ;;
+esac
+export SIXEL_TEST_LDFLAGS
+
+# Skip when the configured compiler or its runtime cannot use ASan.
+printf 'int main(void) { return 0; }\n' >"$SIXEL_TEST_DIR/probe.c"
+if ! make -s -C .. -f Makefile -f regress/image-sixel-buffer.mk \
+    sixel-buffer-probe >"$SIXEL_TEST_DIR/probe.log" 2>&1; then
+	exit 0
+fi
+if ! "$SIXEL_TEST_DIR/probe" >"$SIXEL_TEST_DIR/probe.log" 2>&1; then
+	exit 0
+fi
+
+make -s -C .. -f Makefile -f regress/image-sixel-buffer.mk \
+    sixel-buffer-test || exit 1
+"$SIXEL_TEST_DIR/test" || exit 1