diff --git a/Source/NSMeasurement.m b/Source/NSMeasurement.m
index 437022982..b3bfc79ca 100644
--- a/Source/NSMeasurement.m
+++ b/Source/NSMeasurement.m
@@ -105,7 +105,7 @@
   NSMeasurement *result = nil;
   double v = 0.0;
 
-  v = _doubleValue + [newMeasurement doubleValue];
+  v = _doubleValue - [newMeasurement doubleValue];
   result = [[NSMeasurement alloc] initWithDoubleValue: v unit: _unit];
   AUTORELEASE(result);