diff --git a/source/src/crypto.cpp b/source/src/crypto.cpp
index 44041bed..7380acdd 100644
--- a/source/src/crypto.cpp
+++ b/source/src/crypto.cpp
@@ -849,11 +849,12 @@ bool checkchallenge(const char *answerstr, void *correct)
 #define M (397)
 #define K (0x9908B0DFU)
 
-static uint state[N];
+static uint state[2 * N];
 static int next = N;
 
 void seedMT(uint seed)
 {
+    loopi(N) state[i + N] = state[i];
     state[0] = seed;
     for(uint i = 1; i < N; i++)
         state[i] = seed = 1812433253U * (seed ^ (seed >> 30)) + i;
@@ -877,3 +878,11 @@ uint randomMT()
     return y;
 }
 
+void popMT()  // undo last seedMT()
+{
+    loopi(N) state[i] = state[i + N];
+}
+#undef N
+#undef M
+#undef K
+
diff --git a/source/src/tools.h b/source/src/tools.h
index 59465597..d78544a9 100644
--- a/source/src/tools.h
+++ b/source/src/tools.h
@@ -967,6 +967,7 @@ extern bool isbigendian();
 extern void strtoupper(char *t, const char *s = NULL);
 extern void seedMT(uint seed);
 extern uint randomMT(void);
+extern void popMT(void);
 
 struct iprange { enet_uint32 lr, ur; };
 extern const char *atoip(const char *s, enet_uint32 *ip);
diff --git a/source/src/worldlight.cpp b/source/src/worldlight.cpp
index 63143248..5b5826bf 100644
--- a/source/src/worldlight.cpp
+++ b/source/src/worldlight.cpp
@@ -195,8 +195,7 @@ void calclight()
         s++;
     }
 
-    uint keep = randomMT();
-    seedMT(ents.length() + hdr.maprevision);
+    seedMT(ents.length() + hdr.maprevision);   // static seed -> nothing random here
 
     loopv(ents)
     {
@@ -204,7 +203,7 @@ void calclight()
         if(e.type==LIGHT) calclightsource(e);
     }
 
-    seedMT(keep);
+    popMT();   // undo the static seedMT() from above
 
     block bb = { mapdims.x1 - 1, mapdims.y1 - 1, mapdims.xspan + 2, mapdims.yspan + 2 };
     postlightarea(bb);