diff --git a/src/vmime/security/cert/X509Certificate.hpp b/src/vmime/security/cert/X509Certificate.hpp
index d816646b..edc04816 100644
--- a/src/vmime/security/cert/X509Certificate.hpp
+++ b/src/vmime/security/cert/X509Certificate.hpp
@@ -106,7 +106,7 @@ public:
 	  *
 	  * @return distinguished name of the certificate issuer, as a string
 	  */
-	const string getIssuerString() const;
+	virtual const string getIssuerString() const = 0;
 
 	/** Checks if this certificate has the given issuer.
 	  *