diff --git a/prometheus.scm b/prometheus.scm
index 83061a7..d6caa6a 100644
--- a/prometheus.scm
+++ b/prometheus.scm
@@ -350,6 +350,12 @@ values are the values."
                 '(gauge))
     (error "can only set gauge metrics"))
 
+  (unless (number? value)
+    (raise-exception
+     (make-exception-with-message
+      (simple-format #f "metric-set value must be a number: ~A"
+                     value))))
+
   (call-with-blocked-asyncs
    (lambda ()
      (with-mutex (metric-mutex metric)