diff --git a/src/linkgraph/flowmapper.cpp b/src/linkgraph/flowmapper.cpp
--- a/src/linkgraph/flowmapper.cpp
+++ b/src/linkgraph/flowmapper.cpp
@@ -49,8 +49,10 @@ void FlowMapper::Run(LinkGraphJob &job) 
 		FlowStatMap &flows = node.Flows();
 		flows.FinalizeLocalConsumption(node.Station());
 		if (this->scale) {
-			/* Scale by time the graph has been running without being compressed. */
-			uint runtime = job.JoinDate() - job.Settings().recalc_time - job.LastCompression();
+			/* Scale by time the graph has been running without being compressed. Add 1 to avoid
+			 * division by 0 if spawn date == last compression date. This matches
+			 * LinkGraph::Monthly(). */
+			uint runtime = job.JoinDate() - job.Settings().recalc_time - job.LastCompression() + 1;
 			for (FlowStatMap::iterator i = flows.begin(); i != flows.end(); ++i) {
 				i->second.ScaleToMonthly(runtime);
 			}