diff --git a/test/include/test/test.h b/test/include/test/test.h
index 8cc97af5..a32ec07c 100644
--- a/test/include/test/test.h
+++ b/test/include/test/test.h
@@ -1,13 +1,19 @@
+#define	ASSERT_BUFSIZE	256
+
 #define	assert_cmp(t, a, b, cmp, neg_cmp, pri, fmt...) do {		\
 	t a_ = (a);							\
 	t b_ = (b);							\
 	if (!(a_ cmp b_)) {						\
-		p_test_fail(						\
+		char prefix[ASSERT_BUFSIZE];				\
+		char message[ASSERT_BUFSIZE];				\
+		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
 		    "(%s) "#cmp" (%s) --> "				\
 		    "%"pri" "#neg_cmp" %"pri": ",			\
 		    __func__, __FILE__, __LINE__,			\
-		    #a, #b, a_, b_, fmt);				\
+		    #a, #b, a_, b_);					\
+		malloc_snprintf(message, sizeof(message), fmt);		\
+		p_test_fail(prefix, message);				\
 	}								\
 } while (0)
 
@@ -208,24 +214,32 @@
 	bool a_ = (a);							\
 	bool b_ = (b);							\
 	if (!(a_ == b_)) {						\
-		p_test_fail(						\
+		char prefix[ASSERT_BUFSIZE];				\
+		char message[ASSERT_BUFSIZE];				\
+		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
 		    "(%s) == (%s) --> %s != %s: ",			\
 		    __func__, __FILE__, __LINE__,			\
 		    #a, #b, a_ ? "true" : "false",			\
-		    b_ ? "true" : "false", fmt);			\
+		    b_ ? "true" : "false");				\
+		malloc_snprintf(message, sizeof(message), fmt);		\
+		p_test_fail(prefix, message);				\
 	}								\
 } while (0)
 #define	assert_b_ne(a, b, fmt...) do {					\
 	bool a_ = (a);							\
 	bool b_ = (b);							\
 	if (!(a_ != b_)) {						\
-		p_test_fail(						\
+		char prefix[ASSERT_BUFSIZE];				\
+		char message[ASSERT_BUFSIZE];				\
+		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
 		    "(%s) != (%s) --> %s == %s: ",			\
 		    __func__, __FILE__, __LINE__,			\
 		    #a, #b, a_ ? "true" : "false",			\
-		    b_ ? "true" : "false", fmt);			\
+		    b_ ? "true" : "false");				\
+		malloc_snprintf(message, sizeof(message), fmt);		\
+		p_test_fail(prefix, message);				\
 	}								\
 } while (0)
 #define	assert_true(a, fmt...)	assert_b_eq(a, true, fmt)
@@ -233,26 +247,39 @@
 
 #define	assert_str_eq(a, b, fmt...) do {				\
 	if (strcmp((a), (b))) {						\
-		p_test_fail(						\
+		char prefix[ASSERT_BUFSIZE];				\
+		char message[ASSERT_BUFSIZE];				\
+		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
 		    "(%s) same as (%s) --> "				\
 		    "\"%s\" differs from \"%s\": ",			\
-		    __func__, __FILE__, __LINE__, #a, #b, a, b, fmt);	\
+		    __func__, __FILE__, __LINE__, #a, #b, a, b);	\
+		malloc_snprintf(message, sizeof(message), fmt);		\
+		p_test_fail(prefix, message);				\
 	}								\
 } while (0)
 #define	assert_str_ne(a, b, fmt...) do {				\
 	if (!strcmp((a), (b))) {					\
-		p_test_fail(						\
+		char prefix[ASSERT_BUFSIZE];				\
+		char message[ASSERT_BUFSIZE];				\
+		malloc_snprintf(prefix, sizeof(prefix),			\
 		    "%s:%s:%d: Failed assertion: "			\
 		    "(%s) differs from (%s) --> "			\
 		    "\"%s\" same as \"%s\": ",				\
-		    __func__, __FILE__, __LINE__, #a, #b, a, b, fmt);	\
+		    __func__, __FILE__, __LINE__, #a, #b, a, b);	\
+		malloc_snprintf(message, sizeof(message), fmt);		\
+		p_test_fail(prefix, message);				\
 	}								\
 } while (0)
 
 #define	assert_not_reached(fmt...) do {					\
-	p_test_fail("%s:%s:%d: Unreachable code reached: ",		\
-	    __func__, __FILE__, __LINE__, fmt);				\
+	char prefix[ASSERT_BUFSIZE];					\
+	char message[ASSERT_BUFSIZE];					\
+	malloc_snprintf(prefix, sizeof(prefix),				\
+	    "%s:%s:%d: Unreachable code reached: ",			\
+	    __func__, __FILE__, __LINE__);				\
+	malloc_snprintf(message, sizeof(message), fmt);			\
+	p_test_fail(prefix, message);					\
 } while (0)
 
 /*
@@ -299,4 +326,4 @@ void	test_fail(const char *format, ...) JEMALLOC_ATTR(format(printf, 1, 2));
 test_status_t	p_test(test_t* t, ...);
 void	p_test_init(const char *name);
 void	p_test_fini(void);
-void	p_test_fail(const char *format, ...);
+void	p_test_fail(const char *prefix, const char *message);
diff --git a/test/src/test.c b/test/src/test.c
index 6552e377..528d8583 100644
--- a/test/src/test.c
+++ b/test/src/test.c
@@ -86,15 +86,9 @@ p_test(test_t* t, ...)
 }
 
 void
-p_test_fail(const char *format, ...)
+p_test_fail(const char *prefix, const char *message)
 {
-	va_list ap;
 
-	va_start(ap, format);
-	malloc_vcprintf(NULL, NULL, format, ap);
-	format = va_arg(ap, const char *);
-	malloc_vcprintf(NULL, NULL, format, ap);
-	va_end(ap);
-	malloc_printf("\n");
+	malloc_cprintf(NULL, NULL, "%s%s\n", prefix, message);
 	test_status = test_status_fail;
 }
diff --git a/test/unit/SFMT.c b/test/unit/SFMT.c
index 4805f8e4..c57bd68d 100644
--- a/test/unit/SFMT.c
+++ b/test/unit/SFMT.c
@@ -1576,7 +1576,7 @@ TEST_BEGIN(test_by_array_64)
 	for (i = 0; i < BLOCK_SIZE64; i++) {
 		if (i < COUNT_1) {
 			assert_u64_eq(array64[i], init_by_array_64_expected[i],
-			    "Output mismatch for i=%d");
+			    "Output mismatch for i=%d", i);
 		}
 		r = gen_rand64(ctx);
 		assert_u64_eq(r, array64[i],