diff --git a/tools/editor/editor_brush.cpp b/tools/editor/editor_brush.cpp
index 43414778..f5bf651a 100644
--- a/tools/editor/editor_brush.cpp
+++ b/tools/editor/editor_brush.cpp
@@ -6,13 +6,20 @@ namespace wowee {
 namespace editor {
 
 float EditorBrush::getInfluence(float distance) const {
+    // NaN distance must produce 0 influence — comparisons against NaN
+    // return false, so without this the function would fall through to
+    // the "fully inside" branch and return 1.0 for every queried point.
+    if (!std::isfinite(distance) || !std::isfinite(settings_.radius) ||
+        settings_.radius <= 0.0f) return 0.0f;
     if (distance >= settings_.radius) return 0.0f;
 
     float t = distance / settings_.radius;
-    float innerRadius = 1.0f - settings_.falloff;
+    float falloff = std::clamp(settings_.falloff, 0.0f, 1.0f);
+    float innerRadius = 1.0f - falloff;
     if (t <= innerRadius) return 1.0f;
+    if (falloff <= 0.0f) return 1.0f; // hard edge
 
-    float falloffT = (t - innerRadius) / settings_.falloff;
+    float falloffT = (t - innerRadius) / falloff;
     return 1.0f - (falloffT * falloffT);
 }