diff --git a/maxutils/maxbase/include/maxbase/assert.h b/maxutils/maxbase/include/maxbase/assert.h
index ac0391520..9f1a70ed7 100644
--- a/maxutils/maxbase/include/maxbase/assert.h
+++ b/maxutils/maxbase/include/maxbase/assert.h
@@ -31,9 +31,11 @@ MXB_BEGIN_DECLS
             fprintf(stderr, "debug assert at %s:%d failed: %s\n", (char*)__FILE__, __LINE__, debug_expr); \
             raise(SIGABRT);}} while (false)
 
-#define mxb_assert_message(exp, message) \
+#define mxb_assert_message(exp, fmt, ...) \
     do {if (exp) {} else {     \
             const char* debug_expr = #exp; \
+            char message[1024]; \
+            snprintf(message, sizeof(message), fmt, ##__VA_ARGS__); \
             MXB_ERROR("debug assert at %s:%d failed: %s (%s)\n", \
                       (char*)__FILE__, \
                       __LINE__, \
diff --git a/server/core/config.cc b/server/core/config.cc
index c4525b01a..96f6ee25b 100644
--- a/server/core/config.cc
+++ b/server/core/config.cc
@@ -1966,7 +1966,8 @@ const
     string value = get_string(key);
     milliseconds duration {0};
     MXB_AT_DEBUG(bool rval = ) get_suffixed_duration(value.c_str(), interpretation, &duration);
-    mxb_assert(rval);   // When this function is called, the validity of the value should have been checked.
+    // When this function is called, the validity of the value should have been checked.
+    mxb_assert_message(rval, "Invalid value for '%s': %s", key.c_str(), value.c_str());
     return duration;
 }
 
diff --git a/server/core/test/test_modutil.cc b/server/core/test/test_modutil.cc
index ee3df8c73..7e15a0769 100644
--- a/server/core/test/test_modutil.cc
+++ b/server/core/test/test_modutil.cc
@@ -285,19 +285,20 @@ void test_multiple_sql_packets1()
     mxb_assert_message(complete && head, "Both buffers should have data");
     mxb_assert_message(gwbuf_length(complete) + gwbuf_length(head) + gwbuf_length(quarter)
                        + gwbuf_length(half) == sizeof(resultset),
-                       "25% of data should be available");
+                       "a quarter of data should be available");
 
     quarter = gwbuf_append(gwbuf_append(complete, head), quarter);
     complete = modutil_get_complete_packets(&quarter);
     mxb_assert_message(gwbuf_length(complete) + gwbuf_length(quarter)
                        + gwbuf_length(half) == sizeof(resultset),
-                       "50% of data should be available");
+                       "half of data should be available");
 
     half = gwbuf_append(gwbuf_append(complete, quarter), half);
     complete = modutil_get_complete_packets(&half);
     mxb_assert_message(complete, "Complete should not be NULL");
     mxb_assert_message(half == NULL, "Old buffer should be NULL");
-    mxb_assert_message(gwbuf_length(complete) == sizeof(resultset), "Complete should contain 100% of data");
+    mxb_assert_message(gwbuf_length(complete) == sizeof(resultset),
+                       "Complete should contain all of the data");
 
     completelen = gwbuf_length(complete);
     mxb_assert_message(gwbuf_copy_data(complete, 0, completelen, databuf) == completelen,