diff --git a/libsolidity/interface/CompilerStack.cpp b/libsolidity/interface/CompilerStack.cpp
index 4b7faf496..5292d5329 100644
--- a/libsolidity/interface/CompilerStack.cpp
+++ b/libsolidity/interface/CompilerStack.cpp
@@ -1447,7 +1447,7 @@ CompilerStack::Source const& CompilerStack::source(string const& _sourceName) co
 {
 	auto it = m_sources.find(_sourceName);
 	if (it == m_sources.end())
-		solThrow(CompilerError, "Given source file not found.");
+		solThrow(CompilerError, "Given source file not found: " + _sourceName);
 
 	return it->second;
 }
diff --git a/libsolidity/interface/FileReader.cpp b/libsolidity/interface/FileReader.cpp
index 65cf70e5a..47f4d2b5a 100644
--- a/libsolidity/interface/FileReader.cpp
+++ b/libsolidity/interface/FileReader.cpp
@@ -172,7 +172,7 @@ ReadCallback::Result FileReader::readFile(string const& _kind, string const& _so
 	}
 }
 
-string FileReader::cliPathToSourceUnitName(boost::filesystem::path const& _cliPath)
+string FileReader::cliPathToSourceUnitName(boost::filesystem::path const& _cliPath) const
 {
 	vector<boost::filesystem::path> prefixes = {m_basePath.empty() ? normalizeCLIPathForVFS(".") : m_basePath};
 	prefixes += m_includePaths;
@@ -189,7 +189,7 @@ string FileReader::cliPathToSourceUnitName(boost::filesystem::path const& _cliPa
 	return normalizedPath.generic_string();
 }
 
-map<string, FileReader::FileSystemPathSet> FileReader::detectSourceUnitNameCollisions(FileSystemPathSet const& _cliPaths)
+map<string, FileReader::FileSystemPathSet> FileReader::detectSourceUnitNameCollisions(FileSystemPathSet const& _cliPaths) const
 {
 	map<string, FileReader::FileSystemPathSet> nameToPaths;
 	for (boost::filesystem::path const& cliPath: _cliPaths)
diff --git a/libsolidity/interface/FileReader.h b/libsolidity/interface/FileReader.h
index 71ca5fc5d..3892e9826 100644
--- a/libsolidity/interface/FileReader.h
+++ b/libsolidity/interface/FileReader.h
@@ -93,14 +93,14 @@ public:
 
 	/// Creates a source unit name by normalizing a path given on the command line and, if possible,
 	/// making it relative to base path or one of the include directories.
-	std::string cliPathToSourceUnitName(boost::filesystem::path const& _cliPath);
+	std::string cliPathToSourceUnitName(boost::filesystem::path const& _cliPath) const;
 
 	/// Checks if a set contains any paths that lead to different files but would receive identical
 	/// source unit names. Files are considered the same if their paths are exactly the same after
 	/// normalization (without following symlinks).
 	/// @returns a map containing all the conflicting source unit names and the paths that would
 	/// receive them. The returned paths are normalized.
-	std::map<std::string, FileSystemPathSet> detectSourceUnitNameCollisions(FileSystemPathSet const& _cliPaths);
+	std::map<std::string, FileSystemPathSet> detectSourceUnitNameCollisions(FileSystemPathSet const& _cliPaths) const;
 
 	/// Normalizes a filesystem path to make it include all components up to the filesystem root,
 	/// remove small, inconsequential differences that do not affect the meaning and make it look