Index: applications/editors/josm/plugins/addrinterpolation/src/org/openstreetmap/josm/plugins/AddrInterpolation/AddrInterpolationDialog.java
===================================================================
--- applications/editors/josm/plugins/addrinterpolation/src/org/openstreetmap/josm/plugins/AddrInterpolation/AddrInterpolationDialog.java	(revision 19422)
+++ applications/editors/josm/plugins/addrinterpolation/src/org/openstreetmap/josm/plugins/AddrInterpolation/AddrInterpolationDialog.java	(revision 23102)
@@ -283,6 +283,7 @@
 			public void focusGained(FocusEvent fe){
 				if (!interpolationMethodSet) {
-					AutoDetectInterpolationMethod();
-					interpolationMethodSet = true;  // Don't auto detect over a previous choice
+					if (AutoDetectInterpolationMethod()) {
+						interpolationMethodSet = true;  // Don't auto detect over a previous choice
+					}
 				}
 			}
@@ -343,11 +344,12 @@
 	// Call after both starting and ending housenumbers have been entered - usually when
 	// combo box gets focus.
-	private void AutoDetectInterpolationMethod() {
+	// Return true if a method was detected
+	private boolean AutoDetectInterpolationMethod() {
 
 		String startValueString = ReadTextField(startTextField);
 		String endValueString = ReadTextField(endTextField);
-		if (startValueString.equals("") || endValueString.equals("")) {
+		if ( (startValueString == null) || (endValueString== null) ) {
 			// Not all values entered yet
-			return;
+			return false;
 		}
 
@@ -384,5 +386,5 @@
 				// Both end with alpha
 				SelectInterpolationMethod("alphabetic");
-				return;
+				return true;
 			}
 
@@ -392,10 +394,14 @@
 					// First is a number, last is Latin alpha
 					SelectInterpolationMethod("alphabetic");
-				}
-			}
-
-
-		}
-
+					return true;
+				}
+			}
+			
+			// Did not detect alpha
+			return false;
+
+		}
+		
+		return true;
 
 	}