diff --git a/src/tgp.cpp b/src/tgp.cpp
index 8177b5720c..cf597ea878 100644
--- a/src/tgp.cpp
+++ b/src/tgp.cpp
@@ -667,6 +667,7 @@ static void HeightMapCurves(uint level)
 			for (uint t = 0; t < lengthof(curve_maps); t++) {
 				if (!HasBit(corner_bits, t)) continue;
 
+				bool found = false;
 				const control_point_t *cm = curve_maps[t].list;
 				for (uint i = 0; i < curve_maps[t].length - 1; i++) {
 					const control_point_t &p1 = cm[i];
@@ -674,9 +675,11 @@ static void HeightMapCurves(uint level)
 
 					if (*h >= p1.x && *h < p2.x) {
 						ht[t] = p1.y + (*h - p1.x) * (p2.y - p1.y) / (p2.x - p1.x);
+						found = true;
 						break;
 					}
 				}
+				assert(found);
 			}
 
 			/* Apply interpolation of curve map results. */