diff --git a/GPU/Directx9/VertexShaderGeneratorDX9.cpp b/GPU/Directx9/VertexShaderGeneratorDX9.cpp
index 677bf6c0e5..056bcc1580 100644
--- a/GPU/Directx9/VertexShaderGeneratorDX9.cpp
+++ b/GPU/Directx9/VertexShaderGeneratorDX9.cpp
@@ -604,7 +604,7 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 		bool diffuseIsZero = true;
 		bool specularIsZero = true;
 		bool distanceNeeded = false;
-
+		bool anySpots = false;
 		if (enableLighting) {
 			WRITE(p, "  float4 lightSum0 = u_ambient * %s + float4(u_matemissive, 0.0);\n", ambientStr);
 
@@ -618,6 +618,8 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 					specularIsZero = false;
 				if (type != GE_LIGHTTYPE_DIRECTIONAL)
 					distanceNeeded = true;
+				if (type == GE_LIGHTTYPE_SPOT || type == GE_LIGHTTYPE_UNKNOWN)
+					anySpots = true;
 			}
 
 			if (!specularIsZero) {
@@ -631,6 +633,10 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 				WRITE(p, "  float distance;\n");
 				WRITE(p, "  float lightScale;\n");
 			}
+			WRITE(p, "  float ldot;\n");
+			if (anySpots) {
+				WRITE(p, "  float angle;\n");
+			}
 		}
 
 		// Calculate lights if needed. If shade mapping is enabled, lights may need to be
@@ -654,12 +660,15 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 			bool doSpecular = comp != GE_LIGHTCOMP_ONLYDIFFUSE;
 			bool poweredDiffuse = comp == GE_LIGHTCOMP_BOTHWITHPOWDIFFUSE;
 
+			WRITE(p, "  ldot = max(dot(toLight, worldnormal), 0.0);\n");
 			if (poweredDiffuse) {
-				WRITE(p, "  float dot%i = pow(dot(toLight, worldnormal), u_matspecular.a);\n", i);
-				// TODO: Somehow the NaN check from GLES seems unnecessary here?
-				// If it returned 0, it'd be wrong, so that's strange.
-			} else {
-				WRITE(p, "  float dot%i = dot(toLight, worldnormal);\n", i);
+				// pow(0.0, 0.0) may be undefined, but the PSP seems to treat it as 1.0.
+				// Seen in Tales of the World: Radiant Mythology (#2424.)
+				WRITE(p, "  if (ldot == 0.0 && u_matspecular.a == 0.0) {\n");
+				WRITE(p, "    ldot = 1.0;\n");
+				WRITE(p, "  } else {\n");
+				WRITE(p, "    ldot = pow(ldot, u_matspecular.a);\n");
+				WRITE(p, "  }\n");
 			}
 
 			const char *timesLightScale = " * lightScale";
@@ -674,9 +683,9 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 				break;
 			case GE_LIGHTTYPE_SPOT:
 			case GE_LIGHTTYPE_UNKNOWN:
-				WRITE(p, "  float angle%i = dot(normalize(u_lightdir%i), toLight);\n", i, i);
-				WRITE(p, "  if (angle%i >= u_lightangle_spotCoef%i.x) {\n", i, i);
-				WRITE(p, "    lightScale = clamp(1.0 / dot(u_lightatt%i, float3(1.0, distance, distance*distance)), 0.0, 1.0) * pow(angle%i, u_lightangle_spotCoef%i.y);\n", i, i, i);
+				WRITE(p, "  angle = dot(normalize(u_lightdir%i), toLight);\n", i);
+				WRITE(p, "  if (angle >= u_lightangle_spotCoef%i.x) {\n", i);
+				WRITE(p, "    lightScale = clamp(1.0 / dot(u_lightatt%i, float3(1.0, distance, distance*distance)), 0.0, 1.0) * pow(angle, u_lightangle_spotCoef%i.y);\n", i, i);
 				WRITE(p, "  } else {\n");
 				WRITE(p, "    lightScale = 0.0;\n");
 				WRITE(p, "  }\n");
@@ -686,11 +695,11 @@ void GenerateVertexShaderHLSL(const VShaderID &id, char *buffer, ShaderLanguage
 				break;
 			}
 
-			WRITE(p, "  diffuse = (u_lightdiffuse%i * %s) * max(dot%i, 0.0);\n", i, diffuseStr, i);
+			WRITE(p, "  diffuse = (u_lightdiffuse%i * %s) * ldot;\n", i, diffuseStr);
 			if (doSpecular) {
-				WRITE(p, "  dot%i = dot(normalize(toLight + float3(0.0, 0.0, 1.0)), worldnormal);\n", i);
-				WRITE(p, "  if (dot%i > 0.0)\n", i);
-				WRITE(p, "    lightSum1 += u_lightspecular%i * %s * (pow(dot%i, u_matspecular.a) %s);\n", i, specularStr, i, timesLightScale);
+				WRITE(p, "  ldot = dot(normalize(toLight + float3(0.0, 0.0, 1.0)), worldnormal);\n");
+				WRITE(p, "  if (ldot > 0.0)\n");
+				WRITE(p, "    lightSum1 += u_lightspecular%i * %s * (pow(ldot, u_matspecular.a) %s);\n", i, specularStr, timesLightScale);
 			}
 			WRITE(p, "  lightSum0.rgb += (u_lightambient%i * %s.rgb + diffuse)%s;\n", i, ambientStr, timesLightScale);
 		}