diff --git a/src/de/schildbach/pte/NetworkProviderFactory.java b/src/de/schildbach/pte/NetworkProviderFactory.java
index f454c35e..bacb319a 100644
--- a/src/de/schildbach/pte/NetworkProviderFactory.java
+++ b/src/de/schildbach/pte/NetworkProviderFactory.java
@@ -106,7 +106,9 @@ public final class NetworkProviderFactory
 
 	public static String networkId(final NetworkProvider provider)
 	{
-		if (provider instanceof VbbProvider)
+		if (provider == null)
+			throw new IllegalArgumentException("null provider");
+		else if (provider instanceof VbbProvider)
 			return VbbProvider.NETWORK_ID;
 		else if (provider instanceof RmvProvider)
 			return RmvProvider.NETWORK_ID;