diff --git a/include/gfx/pal_sorting.hpp b/include/gfx/pal_sorting.hpp
index ff4f2958..73957a3c 100644
--- a/include/gfx/pal_sorting.hpp
+++ b/include/gfx/pal_sorting.hpp
@@ -16,7 +16,7 @@ struct Palette;
 namespace sorting {
 
 void indexed(std::vector<Palette> &palettes, int palSize, png_color const *palRGB,
-             png_byte *palAlpha);
+             int palAlphaSize, png_byte *palAlpha);
 void grayscale(std::vector<Palette> &palettes,
                std::array<std::optional<Rgba>, 0x8001> const &colors);
 void rgb(std::vector<Palette> &palettes);
diff --git a/src/gfx/pal_sorting.cpp b/src/gfx/pal_sorting.cpp
index f5df6fbc..6d4c6744 100644
--- a/src/gfx/pal_sorting.cpp
+++ b/src/gfx/pal_sorting.cpp
@@ -14,12 +14,12 @@
 namespace sorting {
 
 void indexed(std::vector<Palette> &palettes, int palSize, png_color const *palRGB,
-             png_byte *palAlpha) {
+             int palAlphaSize, png_byte *palAlpha) {
 	options.verbosePrint(Options::VERB_LOG_ACT, "Sorting palettes using embedded palette...\n");
 
-	auto pngToRgb = [&palRGB, &palAlpha](int index) {
+	auto pngToRgb = [&palRGB, &palAlphaSize, &palAlpha](int index) {
 		auto const &c = palRGB[index];
-		return Rgba(c.red, c.green, c.blue, palAlpha ? palAlpha[index] : 0xFF);
+		return Rgba(c.red, c.green, c.blue, palAlpha && index < palAlphaSize ? palAlpha[index] : 0xFF);
 	};
 
 	for (Palette &pal : palettes) {
diff --git a/src/gfx/process.cpp b/src/gfx/process.cpp
index c3929156..513211b7 100644
--- a/src/gfx/process.cpp
+++ b/src/gfx/process.cpp
@@ -83,6 +83,7 @@ class Png {
 	int colorType;
 	int nbColors;
 	png_colorp embeddedPal = nullptr;
+	int nbTransparentEntries;
 	png_bytep transparencyPal = nullptr;
 
 	[[noreturn]] static void handleError(png_structp png, char const *msg) {
@@ -116,8 +117,8 @@ public:
 
 	int getColorType() const { return colorType; }
 
-	std::tuple<int, png_const_colorp, png_bytep> getEmbeddedPal() const {
-		return {nbColors, embeddedPal, transparencyPal};
+	std::tuple<int, png_const_colorp, int, png_bytep> getEmbeddedPal() const {
+		return {nbColors, embeddedPal, nbTransparentEntries, transparencyPal};
 	}
 
 	uint32_t getWidth() const { return width; }
@@ -257,9 +258,8 @@ public:
 		                     height, bitDepth, colorTypeName(), interlaceTypeName());
 
 		if (png_get_PLTE(png, info, &embeddedPal, &nbColors) != 0) {
-			int nbTransparentEntries;
 			if (png_get_tRNS(png, info, &transparencyPal, &nbTransparentEntries, nullptr)) {
-				assert(nbTransparentEntries == nbColors);
+				assert(nbTransparentEntries <= nbColors);
 			}
 
 			options.verbosePrint(Options::VERB_INTERM, "Embedded palette has %d colors: [",
@@ -268,7 +268,8 @@ public:
 				auto const &color = embeddedPal[i];
 				options.verbosePrint(
 				    Options::VERB_INTERM, "#%02x%02x%02x%02x%s", color.red, color.green, color.blue,
-				    transparencyPal ? transparencyPal[i] : 0xFF, i != nbColors - 1 ? ", " : "]\n");
+				    transparencyPal && i < nbTransparentEntries ? transparencyPal[i] : 0xFF,
+				    i != nbColors - 1 ? ", " : "]\n");
 			}
 		} else {
 			options.verbosePrint(Options::VERB_INTERM, "No embedded palette\n");
@@ -510,7 +511,7 @@ struct AttrmapEntry {
 
 static void generatePalSpec(Png const &png) {
 	// Generate a palette spec from the first few colors in the embedded palette
-	auto [embPalSize, embPalRGB, embPalAlpha] = png.getEmbeddedPal();
+	auto [embPalSize, embPalRGB, embPalAlphaSize, embPalAlpha] = png.getEmbeddedPal();
 	if (embPalRGB == nullptr) {
 		fatal("`-c embedded` was given, but the PNG does not have an embedded palette!");
 	}
@@ -523,7 +524,7 @@ static void generatePalSpec(Png const &png) {
 	}
 	for (int i = 0; i < embPalSize; ++i) {
 		options.palSpec[0][i] = Rgba(embPalRGB[i].red, embPalRGB[i].green, embPalRGB[i].blue,
-		                             embPalAlpha ? embPalAlpha[i] : 0xFF);
+		                             embPalAlpha && i < embPalAlphaSize ? embPalAlpha[i] : 0xFF);
 	}
 }
 
@@ -559,9 +560,9 @@ static std::tuple<DefaultInitVec<size_t>, std::vector<Palette>>
 	}
 
 	// "Sort" colors in the generated palettes, see the man page for the flowchart
-	auto [embPalSize, embPalRGB, embPalAlpha] = png.getEmbeddedPal();
+	auto [embPalSize, embPalRGB, embPalAlphaSize, embPalAlpha] = png.getEmbeddedPal();
 	if (embPalRGB != nullptr) {
-		sorting::indexed(palettes, embPalSize, embPalRGB, embPalAlpha);
+		sorting::indexed(palettes, embPalSize, embPalRGB, embPalAlphaSize, embPalAlpha);
 	} else if (png.isSuitableForGrayscale()) {
 		sorting::grayscale(palettes, png.getColors().raw());
 	} else {
diff --git a/test/gfx/rgbgfx_test.cpp b/test/gfx/rgbgfx_test.cpp
index c1c1bc59..b8c0650a 100644
--- a/test/gfx/rgbgfx_test.cpp
+++ b/test/gfx/rgbgfx_test.cpp
@@ -90,6 +90,7 @@ class Png {
 	int colorType;
 	int nbColors;
 	png_colorp embeddedPal = nullptr;
+	int nbTransparentEntries;
 	png_bytep transparencyPal = nullptr;
 
 	[[noreturn]] static void handleError(png_structp png, char const *msg) {
@@ -186,9 +187,8 @@ public:
 		pixels.resize(static_cast<size_t>(width) * static_cast<size_t>(height));
 
 		if (png_get_PLTE(png, info, &embeddedPal, &nbColors) != 0) {
-			int nbTransparentEntries;
 			if (png_get_tRNS(png, info, &transparencyPal, &nbTransparentEntries, nullptr)) {
-				assert(nbTransparentEntries == nbColors);
+				assert(nbTransparentEntries <= nbColors);
 			}
 		}
 
diff --git a/test/gfx/trns_lt_plte.err b/test/gfx/trns_lt_plte.err
new file mode 100644
index 00000000..e69de29b
diff --git a/test/gfx/trns_lt_plte.flags b/test/gfx/trns_lt_plte.flags
new file mode 100644
index 00000000..2e52e40e
--- /dev/null
+++ b/test/gfx/trns_lt_plte.flags
@@ -0,0 +1 @@
+-Z
diff --git a/test/gfx/trns_lt_plte.png b/test/gfx/trns_lt_plte.png
new file mode 100644
index 00000000..42ed9e81
Binary files /dev/null and b/test/gfx/trns_lt_plte.png differ