Index: task-2.5.1+dfsg/src/recur.cpp
===================================================================
--- task-2.5.1+dfsg.orig/src/recur.cpp
+++ task-2.5.1+dfsg/src/recur.cpp
@@ -268,7 +268,10 @@ ISO8601d getNextRecurrence (ISO8601d& cu
            Lexer::isAllDigits (period.substr (1, period.length () - 2)) &&
            period[period.length () - 1] == 'M')
   {
-    int increment = strtol (period.substr (0, period.length () - 1).c_str (), NULL, 10);
+    int increment = strtol (period.substr (1, period.length () - 2).c_str (), NULL, 10);
+
+    if (increment <= 0)
+      throw std::string (format (STRING_TASK_VALID_RECUR, period));
 
     m += increment;
     while (m > 12)