diff --git a/docs/rh-log.txt b/docs/rh-log.txt
index e2330b264..835cc4d3d 100644
--- a/docs/rh-log.txt
+++ b/docs/rh-log.txt
@@ -1,4 +1,7 @@
 September 10, 2009
+- Fixed: If a damaged actor has negative mass, it needs to have its damage
+  thrust set to maximum instead of 0. See Action Doom's broken glass
+  "CommanderKeens".
 - Changed the SCROLLTYPE define so that any extra bits set cause it to default
   to 7. This effects action.wad, MAP02, lines 12054 and 12059, which had them
   at 128, so they no longer scrolled in r832+.
diff --git a/src/p_interaction.cpp b/src/p_interaction.cpp
index c3d38d190..a0d53c1f5 100644
--- a/src/p_interaction.cpp
+++ b/src/p_interaction.cpp
@@ -1047,7 +1047,13 @@ void P_DamageMobj (AActor *target, AActor *inflictor, AActor *source, int damage
 
 			// Calculate this as float to avoid overflows so that the
 			// clamping that had to be done here can be removed.
-			double fltthrust = clamp((damage * 0.125 * kickback) / target->Mass, 0.,mod == NAME_MDK? 10. : 32.);
+			double fltthrust;
+
+			fltthrust = mod == NAME_MDK ? 10 : 32;
+			if (target->Mass > 0)
+			{
+				fltthrust = clamp((damage * 0.125 * kickback) / target->Mass, 0., fltthrust);
+			}
 
 			thrust = FLOAT2FIXED(fltthrust);