diff --git a/tools/editor/terrain_editor.cpp b/tools/editor/terrain_editor.cpp
index 2a684807..d1bc48ec 100644
--- a/tools/editor/terrain_editor.cpp
+++ b/tools/editor/terrain_editor.cpp
@@ -785,11 +785,16 @@ void TerrainEditor::mirrorY() {
 void TerrainEditor::carveRiver(const glm::vec3& start, const glm::vec3& end,
                                 float width, float depth) {
     if (!terrain_) return;
+    if (!std::isfinite(start.x) || !std::isfinite(start.y) ||
+        !std::isfinite(end.x) || !std::isfinite(end.y) ||
+        !std::isfinite(width) || width <= 0.0f ||
+        !std::isfinite(depth)) return;
     recordGeneratorUndo();
     glm::vec2 lineStart(start.x, start.y);
     glm::vec2 lineEnd(end.x, end.y);
-    glm::vec2 lineDir = glm::normalize(lineEnd - lineStart);
     float lineLen = glm::length(lineEnd - lineStart);
+    if (lineLen < 1e-4f) return;
+    glm::vec2 lineDir = (lineEnd - lineStart) / lineLen;
 
     for (int ci = 0; ci < 256; ci++) {
         auto& chunk = terrain_->chunks[ci];
@@ -1370,11 +1375,16 @@ void TerrainEditor::createIsland(float centerHeight, float edgeDropoff) {
 void TerrainEditor::createRidge(const glm::vec3& start, const glm::vec3& end,
                                  float width, float height) {
     if (!terrain_) return;
+    if (!std::isfinite(start.x) || !std::isfinite(start.y) ||
+        !std::isfinite(end.x) || !std::isfinite(end.y) ||
+        !std::isfinite(width) || width <= 0.0f ||
+        !std::isfinite(height)) return;
     recordGeneratorUndo();
     glm::vec2 lineStart(start.x, start.y);
     glm::vec2 lineEnd(end.x, end.y);
-    glm::vec2 lineDir = glm::normalize(lineEnd - lineStart);
     float lineLen = glm::length(lineEnd - lineStart);
+    if (lineLen < 1e-4f) return;
+    glm::vec2 lineDir = (lineEnd - lineStart) / lineLen;
 
     for (int ci = 0; ci < 256; ci++) {
         auto& chunk = terrain_->chunks[ci];
@@ -1406,11 +1416,17 @@ void TerrainEditor::createRidge(const glm::vec3& start, const glm::vec3& end,
 
 void TerrainEditor::flattenRoad(const glm::vec3& start, const glm::vec3& end, float width) {
     if (!terrain_) return;
+    if (!std::isfinite(start.x) || !std::isfinite(start.y) || !std::isfinite(start.z) ||
+        !std::isfinite(end.x) || !std::isfinite(end.y) || !std::isfinite(end.z) ||
+        !std::isfinite(width) || width <= 0.0f) return;
     recordGeneratorUndo();
     glm::vec2 lineStart(start.x, start.y);
     glm::vec2 lineEnd(end.x, end.y);
-    glm::vec2 lineDir = glm::normalize(lineEnd - lineStart);
     float lineLen = glm::length(lineEnd - lineStart);
+    // Zero-length road would normalize to NaN and propagate NaN through
+    // every dist comparison — paint the height delta on every vertex.
+    if (lineLen < 1e-4f) return;
+    glm::vec2 lineDir = (lineEnd - lineStart) / lineLen;
 
     // Interpolate height along the path
     auto heightAtT = [&](float t) -> float {