diff --git a/libyul/backends/wasm/BinaryTransform.cpp b/libyul/backends/wasm/BinaryTransform.cpp
index 58a34ac04..87f47e28c 100644
--- a/libyul/backends/wasm/BinaryTransform.cpp
+++ b/libyul/backends/wasm/BinaryTransform.cpp
@@ -305,13 +305,15 @@ bytes BinaryTransform::run(Module const& _module)
 	ret += exportSection(functionIDs);
 
 	map<string, pair<size_t, size_t>> subModulePosAndSize;
-	for (auto const& sub: _module.subModules)
+	for (auto const& [name, module]: _module.subModules)
 	{
 		// TODO should we prefix and / or shorten the name?
-		bytes data = BinaryTransform::run(sub.second);
-		size_t length = data.size();
-		ret += customSection(sub.first, move(data));
-		subModulePosAndSize[sub.first] = {ret.size() - length, length};
+		bytes data = BinaryTransform::run(module);
+		size_t const length = data.size();
+		ret += customSection(name, move(data));
+		// Skip all the previous sections and the size field of this current custom section.
+		size_t const offset = ret.size() - length;
+		subModulePosAndSize[name] = {offset, length};
 	}
 
 	BinaryTransform bt(
diff --git a/libyul/backends/wasm/BinaryTransform.h b/libyul/backends/wasm/BinaryTransform.h
index 348250546..d164c11bf 100644
--- a/libyul/backends/wasm/BinaryTransform.h
+++ b/libyul/backends/wasm/BinaryTransform.h
@@ -112,6 +112,8 @@ private:
 	std::map<std::string, size_t> const m_globalIDs;
 	std::map<std::string, size_t> const m_functionIDs;
 	std::map<std::string, size_t> const m_functionTypes;
+	/// The map of submodules, where the pair refers to the [offset, length]. The offset is
+	/// an absolute offset within the resulting assembled bytecode.
 	std::map<std::string, std::pair<size_t, size_t>> const m_subModulePosAndSize;
 
 	std::map<std::string, size_t> m_locals;