diff --git a/bin/varnishtest/vtc_log.c b/bin/varnishtest/vtc_log.c
index 5fa44e7bee5..67aea1de31b 100644
--- a/bin/varnishtest/vtc_log.c
+++ b/bin/varnishtest/vtc_log.c
@@ -40,6 +40,7 @@
 #include "vtc_log.h"
 
 #include "vtim.h"
+#include "vbt.h"
 
 static pthread_mutex_t	vtclog_mtx;
 static char		*vtclog_buf;
@@ -296,20 +297,30 @@ static void v_noreturn_
 vtc_log_VAS_Fail(const char *func, const char *file, int line,
     const char *cond, enum vas_e why)
 {
+	char buf[4096] = "";
 	struct vtclog *vl;
 	int e = errno;
 
 	(void)why;
+
+	if (VBT_dump(sizeof buf, buf) < 0) {
+		bprintf(buf, "Failed to print backtrace: %d (%s)\n",
+		    errno, strerror(errno));
+	}
+
 	vl = pthread_getspecific(log_key);
 	if (vl == NULL || vl->act) {
 		fprintf(stderr,
 		    "Assert error in %s(), %s line %d:\n"
-		    "  Condition(%s) not true. (errno=%d %s)\n",
-		    func, file, line, cond, e, strerror(e));
+		    "  Condition(%s) not true. (errno=%d %s)\n"
+		    "%s\n",
+		    func, file, line, cond, e, strerror(e), buf);
 	} else
 		vtc_fatal(vl, "Assert error in %s(), %s line %d:"
 		    "  Condition(%s) not true."
-		    "  Errno=%d %s", func, file, line, cond, e, strerror(e));
+		    "  Errno=%d %s\n"
+		    "%s\n",
+		    func, file, line, cond, e, strerror(e), buf);
 	abort();
 }
 
diff --git a/include/vbt.h b/include/vbt.h
index bd3a384b937..c2c8b28f2c4 100644
--- a/include/vbt.h
+++ b/include/vbt.h
@@ -31,3 +31,4 @@
 struct vsb;
 
 void VBT_format(struct vsb *);
+int VBT_dump(size_t len, char [len]);
diff --git a/lib/libvarnish/vas.c b/lib/libvarnish/vas.c
index b64418cc956..c45202c3c2f 100644
--- a/lib/libvarnish/vas.c
+++ b/lib/libvarnish/vas.c
@@ -38,6 +38,7 @@
 #include <string.h>
 
 #include "vdef.h"
+#include "vbt.h"
 
 #include "vas.h"
 
@@ -57,37 +58,53 @@ VAS_errtxt(int e)
 
 vas_f *VAS_Fail_Func v_noreturn_;
 
-void v_noreturn_
-VAS_Fail(const char *func, const char *file, int line,
+static void
+vas_default(const char *func, const char *file, int line,
     const char *cond, enum vas_e kind)
 {
 	int err = errno;
+	char buf[4096];
 
-	if (VAS_Fail_Func != NULL) {
-		VAS_Fail_Func(func, file, line, cond, kind);
+	if (kind == VAS_MISSING) {
+		fprintf(stderr,
+		    "Missing error handling code in %s(), %s line %d:\n"
+		    "  Condition(%s) not true.\n",
+		    func, file, line, cond);
+	} else if (kind == VAS_INCOMPLETE) {
+		fprintf(stderr,
+		    "Incomplete code in %s(), %s line %d:\n",
+		    func, file, line);
+	} else if (kind == VAS_WRONG) {
+		fprintf(stderr,
+		    "Wrong turn in %s(), %s line %d: %s\n",
+		    func, file, line, cond);
 	} else {
-		if (kind == VAS_MISSING) {
-			fprintf(stderr,
-			    "Missing error handling code in %s(), %s line %d:\n"
-			    "  Condition(%s) not true.\n",
-			    func, file, line, cond);
-		} else if (kind == VAS_INCOMPLETE) {
-			fprintf(stderr,
-			    "Incomplete code in %s(), %s line %d:\n",
-			    func, file, line);
-		} else if (kind == VAS_WRONG) {
-			fprintf(stderr,
-			    "Wrong turn in %s(), %s line %d: %s\n",
-			    func, file, line, cond);
-		} else {
-			fprintf(stderr,
-			    "Assert error in %s(), %s line %d:\n"
-			    "  Condition(%s) not true.\n",
-			    func, file, line, cond);
-		}
-		if (err)
-			fprintf(stderr,
-			    "  errno = %d (%s)\n", err, strerror(err));
+		fprintf(stderr,
+		    "Assert error in %s(), %s line %d:\n"
+		    "  Condition(%s) not true.\n",
+		    func, file, line, cond);
+	}
+	if (err) {
+		fprintf(stderr,
+		    "  errno = %d (%s)\n", err, strerror(err));
 	}
+
+	if (VBT_dump(sizeof buf, buf) < 0) {
+		bprintf(buf, "Failed to print backtrace: %d (%s)\n",
+		    errno, strerror(errno));
+	}
+
+	fprintf(stderr, "%s", buf);
+}
+
+void v_noreturn_
+VAS_Fail(const char *func, const char *file, int line,
+    const char *cond, enum vas_e kind)
+{
+
+	if (VAS_Fail_Func != NULL)
+		VAS_Fail_Func(func, file, line, cond, kind);
+	else
+		vas_default(func, file, line, cond, kind);
 	abort();
 }
diff --git a/lib/libvarnish/vbt.c b/lib/libvarnish/vbt.c
index f68906e51ee..27225d732ae 100644
--- a/lib/libvarnish/vbt.c
+++ b/lib/libvarnish/vbt.c
@@ -150,3 +150,19 @@ VBT_format(struct vsb *vsb)
 	vbt_execinfo(vsb);
 #endif
 }
+
+int
+VBT_dump(size_t len, char buf[len])
+{
+	struct vsb vsb[1];
+
+	if (buf == NULL || VSB_init(vsb, buf, len) == NULL)
+		return (-1);
+
+	VSB_printf(vsb, "Backtrace:\n");
+	VSB_indent(vsb, 2);
+	VBT_format(vsb);
+	VSB_indent(vsb, -2);
+
+	return (0);
+}