diff --git a/src/main/clj/org/domaindrivenarchitecture/activity_pub_poc/core.clj b/src/main/clj/org/domaindrivenarchitecture/activity_pub_poc/core.clj
index 2a4b39d..b41b2c3 100644
--- a/src/main/clj/org/domaindrivenarchitecture/activity_pub_poc/core.clj
+++ b/src/main/clj/org/domaindrivenarchitecture/activity_pub_poc/core.clj
@@ -27,13 +27,16 @@
 (def parsed-rdf-syntax (parse (slurp cm/activitystreams-ttl)))
 
 (defn type-string? [input]
-  (re-matches #"[A-Z]" (str (first input))))
+  (and (string? input)
+       (re-matches #"[A-Z]" (str (first input)))))
 
 (defn url-string? [input]
-  (re-matches #"https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{2,256}\.[a-z]{2,4}\b([-a-zA-Z0-9@:%_\+.~#?&//=]*)" input))
+  (and (string? input)
+       (re-matches #"https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{2,256}\.[a-z]{2,4}\b([-a-zA-Z0-9@:%_\+.~#?&//=]*)" input)))
 
 (defn uri-string? [input]
-  (re-matches #"\w+:(\/?\/?)[^\s]+" input))
+  (and (string? input)
+       (re-matches #"\w+:(\/?\/?)[^\s]+" input)))
 
 (defn uri-vector? [input]
   (and (every? #(uri-string? %) input)