diff --git a/Makefile b/Makefile
index ff2c0db8..6e65f219 100644
--- a/Makefile
+++ b/Makefile
@@ -104,7 +104,7 @@ amy-message: $(OBJECTS) src/amy-message.o
 
 # Plain C tests for things the audio-rendering suite can't reach -- e.g. clock
 # rollovers 50 days out, which you can only hit by fast-forwarding the counters.
-CTESTS = tests/test_clock_wrap
+CTESTS = tests/test_clock_wrap tests/test_sequencer_bounds
 
 # Static pattern rules, so these win over the generic %.o: %.c above (which
 # would compile without -Isrc and fail to find amy.h).
diff --git a/src/sequencer.c b/src/sequencer.c
index 46a20a09..80bb98c5 100644
--- a/src/sequencer.c
+++ b/src/sequencer.c
@@ -182,7 +182,15 @@ uint8_t sequencer_add_event(amy_event *e) {
     // if the tag already exists - if there's tick/period, overwrite, if there's no tick / period, we should remove the entry
     //fprintf(stderr, "sequencer_add_event: e->instrument %d e->note %.0f e->vel %.2f tick %d period %d tag %d\n", e->instrument, e->midi_note, e->velocity, e->sequence[SEQUENCE_TICK], e->sequence[SEQUENCE_PERIOD], e->sequence[SEQUENCE_TAG]);
     int32_t tag = e->sequence[SEQUENCE_TAG];
-    if (tag > max_sequences) {
+    // `sequences` is max_sequences entries, so the last valid tag is
+    // max_sequences - 1. This read `>` and let tag == max_sequences
+    // through to write one sequence_info_t past the end of the
+    // allocation -- the message the check prints has always said "greater
+    // than or eq", which is what it was meant to be doing. A negative tag
+    // is rejected here too: SEQUENCE_TAG arrives as an unsigned field but
+    // lands in an int32_t, so a value past INT32_MAX reads as negative and
+    // would index backwards out of the array.
+    if (tag < 0 || tag >= max_sequences) {
         fprintf(stderr, "sequencer tag %" PRIi32" (with tick %" PRIu32", period %" PRIu32") is greater than or eq max_sequences %" PRIi32"\n",
                 tag, e->sequence[SEQUENCE_TICK], e->sequence[SEQUENCE_PERIOD], max_sequences);
         // ignore
diff --git a/tests/test_sequencer_bounds.c b/tests/test_sequencer_bounds.c
new file mode 100644
index 00000000..f8146901
--- /dev/null
+++ b/tests/test_sequencer_bounds.c
@@ -0,0 +1,102 @@
+// Regression test for the sequencer tag bounds check.
+//
+// `sequences` is a malloc'd array of max_sequences entries, so the last
+// valid tag is max_sequences - 1. sequencer_add_event() guarded with
+//
+//     if (tag > max_sequences)
+//
+// which let tag == max_sequences through and wrote a whole
+// sequence_info_t (a pointer and two uint32_t) one element past the end
+// of the allocation. The message it prints on rejection has always said
+// "greater than or eq", so the intent was never in doubt.
+//
+// Not reachable from the audio-rendering suite, which never sends a tag
+// near the ceiling -- and not reachable from a well-behaved host either,
+// since an allocator that hands out tags below the limit can't produce
+// one. It takes exactly one hand-written message:
+//
+//     amy.send(sequence="0,16,256")     # with max_sequencer_tags 256
+//
+// Build/run with `make ctest`.
+
+#include <stdio.h>
+#include <stdint.h>
+#include <inttypes.h>
+#include "amy.h"
+#include "sequencer.h"
+
+static int failures = 0;
+
+#define CHECK(cond, fmt, ...) do {                                        \
+    if (cond) { printf("  ok   " fmt "\n", ##__VA_ARGS__); }              \
+    else { printf("  FAIL " fmt "\n", ##__VA_ARGS__); failures++; }       \
+} while (0)
+
+#define MAX_TAGS 64
+
+// A schedulable event: a period is what makes sequencer_add_event keep it
+// rather than returning 0 for "nothing to schedule here".
+static uint8_t add_at_tag(int32_t tag) {
+    amy_event e = amy_default_event();
+    e.osc = 0;
+    e.wave = SINE;
+    e.velocity = 1.0f;
+    e.midi_note = 60;
+    e.sequence[SEQUENCE_TICK] = 0;
+    e.sequence[SEQUENCE_PERIOD] = 16;
+    e.sequence[SEQUENCE_TAG] = (uint32_t)tag;
+    return sequencer_add_event(&e);
+}
+
+static void test_tag_bounds(void) {
+    printf("sequencer tag bounds (max_sequences = %d)\n", MAX_TAGS);
+
+    CHECK(add_at_tag(0) == 1, "tag 0 is accepted");
+    CHECK(add_at_tag(MAX_TAGS - 1) == 1,
+          "tag max-1 (%d) is accepted -- the last valid slot", MAX_TAGS - 1);
+
+    // The bug: this one used to be written, one element past the end.
+    CHECK(add_at_tag(MAX_TAGS) == 0,
+          "tag max (%d) is REJECTED, not written past the end", MAX_TAGS);
+    CHECK(add_at_tag(MAX_TAGS + 1) == 0, "tag max+1 is rejected");
+    CHECK(add_at_tag(1000000) == 0, "a far-out tag is rejected");
+
+    // SEQUENCE_TAG is an unsigned field read into an int32_t, so anything
+    // past INT32_MAX arrives negative and would index backwards.
+    CHECK(add_at_tag((int32_t)0x80000000u) == 0,
+          "a tag that reads negative is rejected");
+}
+
+// ...and it still refuses with every legitimate slot occupied, which is
+// the state a host that fills its allocation is actually in.
+static void test_full_table(void) {
+    printf("a full table still refuses the one past the end\n");
+
+    sequencer_reset();
+    int filled = 0;
+    for (int32_t tag = 0; tag < MAX_TAGS; ++tag)
+        filled += add_at_tag(tag);
+    CHECK(filled == MAX_TAGS, "every one of the %d valid tags took an event",
+          MAX_TAGS);
+    CHECK(add_at_tag(MAX_TAGS) == 0, "tag %d is still rejected", MAX_TAGS);
+}
+
+// examples.c calls this; the platform normally provides it.
+void delay_ms(uint32_t ms) { (void)ms; }
+
+int main(void) {
+    amy_config_t c = amy_default_config();
+    c.features.startup_bleep = 0;
+    c.max_sequencer_tags = MAX_TAGS;
+    amy_start(c);
+
+    test_tag_bounds();
+    test_full_table();
+
+    if (failures) {
+        printf("\n%d check(s) FAILED\n", failures);
+        return 1;
+    }
+    printf("\nall sequencer bounds checks passed\n");
+    return 0;
+}