diff --git a/histream-i2b2/src/main/java/de/sekmi/histream/i2b2/ont/Import.java b/histream-i2b2/src/main/java/de/sekmi/histream/i2b2/ont/Import.java
index 9f9c11c49d237bf8b07716f0b0491525fa744d02..0ce38837703196e378733ccb2d09510b56e0c02a 100644
--- a/histream-i2b2/src/main/java/de/sekmi/histream/i2b2/ont/Import.java
+++ b/histream-i2b2/src/main/java/de/sekmi/histream/i2b2/ont/Import.java
@@ -360,7 +360,7 @@ public class Import implements AutoCloseable{
 				done = true;
 			}
 			// skip the http:// part of url prefixes
-			if( path.substring(pos,pos+7).equals("http://") ){
+			if( path.length()-pos > 7 && path.substring(pos,pos+7).equals("http://") ){
 				// prefixes will end in #
 				prefix_char = '#';
 				pos += 7;