diff --git a/RELEASES.rst b/RELEASES.rst
index 0a4f81c1e5..ac313ba2e1 100644
--- a/RELEASES.rst
+++ b/RELEASES.rst
@@ -3021,6 +3021,9 @@ Planned
   without throwing, as internals are not always expecting to deal with an
   error when reading current time (GH-1666)
 
+* Add assertion coverage for catching calls into Duktape API from debug
+  transport calls (read, peek, etc) (GH-1681)
+
 * Fix incorrect handling of register bound unary operation target for
   unary minus, unary plus, and bitwise NOT (GH-1623, GH-1624)
 
diff --git a/src-input/duk_debugger.c b/src-input/duk_debugger.c
index 885d65365d..00ae93f8cd 100644
--- a/src-input/duk_debugger.c
+++ b/src-input/duk_debugger.c
@@ -6,6 +6,24 @@
 
 #if defined(DUK_USE_DEBUGGER_SUPPORT)
 
+/*
+ *  Assert helpers
+ */
+
+#if defined(DUK_USE_ASSERTIONS)
+#define DUK__DBG_TPORT_ENTER() do { \
+		DUK_ASSERT(heap->dbg_calling_transport == 0); \
+		heap->dbg_calling_transport = 1; \
+	} while (0)
+#define DUK__DBG_TPORT_EXIT() do { \
+		DUK_ASSERT(heap->dbg_calling_transport == 1); \
+		heap->dbg_calling_transport = 0; \
+	} while (0)
+#else
+#define DUK__DBG_TPORT_ENTER() do {} while (0)
+#define DUK__DBG_TPORT_EXIT() do {} while (0)
+#endif
+
 /*
  *  Helper structs
  */
@@ -147,6 +165,7 @@ DUK_LOCAL void duk__debug_null_most_callbacks(duk_hthread *thr) {
 
 DUK_INTERNAL duk_bool_t duk_debug_read_peek(duk_hthread *thr) {
 	duk_heap *heap;
+	duk_bool_t ret;
 
 	DUK_ASSERT(thr != NULL);
 	heap = thr->heap;
@@ -161,7 +180,10 @@ DUK_INTERNAL duk_bool_t duk_debug_read_peek(duk_hthread *thr) {
 		return 0;
 	}
 
-	return (duk_bool_t) (heap->dbg_peek_cb(heap->dbg_udata) > 0);
+	DUK__DBG_TPORT_ENTER();
+	ret = (duk_bool_t) (heap->dbg_peek_cb(heap->dbg_udata) > 0);
+	DUK__DBG_TPORT_EXIT();
+	return ret;
 }
 
 DUK_INTERNAL void duk_debug_read_flush(duk_hthread *thr) {
@@ -180,7 +202,9 @@ DUK_INTERNAL void duk_debug_read_flush(duk_hthread *thr) {
 		return;
 	}
 
+	DUK__DBG_TPORT_ENTER();
 	heap->dbg_read_flush_cb(heap->dbg_udata);
+	DUK__DBG_TPORT_EXIT();
 }
 
 DUK_INTERNAL void duk_debug_write_flush(duk_hthread *thr) {
@@ -199,7 +223,9 @@ DUK_INTERNAL void duk_debug_write_flush(duk_hthread *thr) {
 		return;
 	}
 
+	DUK__DBG_TPORT_ENTER();
 	heap->dbg_write_flush_cb(heap->dbg_udata);
+	DUK__DBG_TPORT_EXIT();
 }
 
 /*
@@ -277,7 +303,10 @@ DUK_INTERNAL void duk_debug_read_bytes(duk_hthread *thr, duk_uint8_t *data, duk_
 #if defined(DUK_USE_DEBUGGER_TRANSPORT_TORTURE)
 		left = 1;
 #endif
+		DUK__DBG_TPORT_ENTER();
 		got = heap->dbg_read_cb(heap->dbg_udata, (char *) p, left);
+		DUK__DBG_TPORT_EXIT();
+
 		if (got == 0 || got > left) {
 			DUK_D(DUK_DPRINT("connection error during read, return zero data"));
 			duk__debug_null_most_callbacks(thr);  /* avoid calling write callback in detach1() */
@@ -634,7 +663,10 @@ DUK_INTERNAL void duk_debug_write_bytes(duk_hthread *thr, const duk_uint8_t *dat
 #if defined(DUK_USE_DEBUGGER_TRANSPORT_TORTURE)
 		left = 1;
 #endif
+		DUK__DBG_TPORT_ENTER();
 		got = heap->dbg_write_cb(heap->dbg_udata, (const char *) p, left);
+		DUK__DBG_TPORT_EXIT();
+
 		if (got == 0 || got > left) {
 			duk__debug_null_most_callbacks(thr);  /* avoid calling write callback in detach1() */
 			DUK_D(DUK_DPRINT("connection error during write"));
diff --git a/src-input/duk_heap.h b/src-input/duk_heap.h
index e5006b2f83..cc0aaf8066 100644
--- a/src-input/duk_heap.h
+++ b/src-input/duk_heap.h
@@ -527,6 +527,9 @@ struct duk_heap {
 	/* Used to support single-byte stream lookahead. */
 	duk_bool_t dbg_have_next_byte;
 	duk_uint8_t dbg_next_byte;
+#endif  /* DUK_USE_DEBUGGER_SUPPORT */
+#if defined(DUK_USE_ASSERTIONS)
+	duk_bool_t dbg_calling_transport;       /* transport call in progress, calling into Duktape forbidden */
 #endif
 
 	/* String intern table (weak refs). */
diff --git a/src-input/duk_hthread.h b/src-input/duk_hthread.h
index ccb3025cae..1088c98547 100644
--- a/src-input/duk_hthread.h
+++ b/src-input/duk_hthread.h
@@ -175,6 +175,8 @@
  */
 #define DUK_ASSERT_API_ENTRY(thr) do { \
 		DUK_ASSERT_CTX_VALID((thr)); \
+		DUK_ASSERT((thr)->heap != NULL); \
+		DUK_ASSERT((thr)->heap->dbg_calling_transport == 0); \
 	} while (0)
 
 /*