diff --git a/qbsp/solidbsp.cc b/qbsp/solidbsp.cc
index c6c2030c..915adc98 100644
--- a/qbsp/solidbsp.cc
+++ b/qbsp/solidbsp.cc
@@ -32,6 +32,28 @@ static bool usemidsplit;
 
 //============================================================================
 
+void
+ConvertNodeToLeaf(node_t *node, int contents)
+{
+    // backup the mins/maxs
+    vec3_t mins, maxs;
+    VectorCopy(node->mins, mins);
+    VectorCopy(node->maxs, maxs);
+
+    // zero it
+    memset(node, 0, sizeof(*node));
+    
+    // restore relevant fields
+    VectorCopy(mins, node->mins);
+    VectorCopy(maxs, node->maxs);
+    
+    node->planenum = PLANENUM_LEAF;
+    node->contents = contents;
+    node->markfaces = (face_t **)AllocMem(OTHER, sizeof(face_t *), true);
+
+    Q_assert(node->markfaces[0] == nullptr);
+}
+
 void
 DetailToSolid(node_t *node)
 {
@@ -46,6 +68,15 @@ DetailToSolid(node_t *node)
     } else {
         DetailToSolid(node->children[0]);
         DetailToSolid(node->children[1]);
+
+		// If both children are solid, we can merge the two leafs into one.
+        // DarkPlaces has an assertion that fails if both children are
+        // solid.
+		if (node->children[0]->contents == CONTENTS_SOLID
+			&& node->children[1]->contents == CONTENTS_SOLID) {
+            // This discards any faces on-node. Should be safe (?)
+            ConvertNodeToLeaf(node, CONTENTS_SOLID);
+		}
     }
 }
 
diff --git a/qbsp/writebsp.cc b/qbsp/writebsp.cc
index d4457d35..51190a3f 100644
--- a/qbsp/writebsp.cc
+++ b/qbsp/writebsp.cc
@@ -540,6 +540,13 @@ ExportDrawNodes_BSP29(mapentity_t *entity, node_t *node)
             ExportDrawNodes_BSP29(entity, node->children[i]);
         }
     }
+
+    // DarkPlaces asserts that the leaf numbers are different
+    // if mod_bsp_portalize is 1 (default)
+    // The most likely way it could fail is if both sides are the
+    // shared CONTENTS_SOLID leaf (-1)
+    Q_assert(!(dnode->children[0] == -1 && dnode->children[1] == -1));
+	Q_assert(dnode->children[0] != dnode->children[1]);
 }
 
 static void
@@ -579,6 +586,9 @@ ExportDrawNodes_BSP2(mapentity_t *entity, node_t *node)
             ExportDrawNodes_BSP2(entity, node->children[i]);
         }
     }
+    
+    Q_assert(!(dnode->children[0] == -1 && dnode->children[1] == -1));
+    Q_assert(dnode->children[0] != dnode->children[1]);
 }
 
 static void
@@ -618,6 +628,9 @@ ExportDrawNodes_BSP2rmq(mapentity_t *entity, node_t *node)
             ExportDrawNodes_BSP2rmq(entity, node->children[i]);
         }
     }
+    
+    Q_assert(!(dnode->children[0] == -1 && dnode->children[1] == -1));
+    Q_assert(dnode->children[0] != dnode->children[1]);
 }
 
 /*