diff --git a/source/app.d b/source/app.d
index 604f62d..db49108 100644
--- a/source/app.d
+++ b/source/app.d
@@ -36,7 +36,7 @@ void main(string[] args)
              "mode" : "direct",
              "type_dm" : "any",
              "itdLPxx_bcl" : "true"]);
-    
+
     if (substitutionFileName.exists && substitutionFileName.isFile)
     {
         loadSubstitutionFile(substitutionFileName);
diff --git a/source/substitution.d b/source/substitution.d
index 9275705..deb90b6 100644
--- a/source/substitution.d
+++ b/source/substitution.d
@@ -1,16 +1,65 @@
 module substitution;
 
+import std.file: slurp;
+
 public:
 
-void loadSubstitutionFile(string fileName)
+void loadSubstitutionFile(alias slurpFun = slurp)(string fileName)
 {
-    import std.file : slurp;
     import std.algorithm.iteration : each;
-    auto data = slurp!(string, string)(fileName, `"%s" = "%s"`);
+    auto data = slurpFun!(string, string)(fileName, `"%s" = "%s"`);
     map = (string[string]).init;
     data.each!(pair => map[pair[0]] = pair[1]);
 }
 
+@safe unittest
+{
+    import std.algorithm: canFind;
+    import std.typecons: Tuple, tuple;
+
+    static Tuple!(string, string)[] mockSlurpEmpty(Type1, Type2)(string filename, in char[] format)
+    {
+        return [];
+    }
+
+    loadSubstitutionFile!mockSlurpEmpty("");
+    assert(map.length == 0);
+
+    static Tuple!(string, string)[] mockSlurpEmptyEntry(Type1, Type2)(string filename, in char[] format)
+    {
+        return [tuple("", "")];
+    }
+
+    loadSubstitutionFile!mockSlurpEmptyEntry("");
+    assert("" in map);
+    assert(map.length == 1);
+    assert(map[""] == "");
+
+    static Tuple!(string, string)[] mockSlurpSingleEntry(Type1, Type2)(string filename, in char[] format)
+    {
+        return [tuple("foo", "bar")];
+    }
+
+    loadSubstitutionFile!mockSlurpSingleEntry("");
+    assert("foo" in map);
+    assert(map.length == 1);
+    assert(map["foo"] == "bar");
+
+    static Tuple!(string, string)[] mockSlurpMultipleEntries(Type1, Type2)(string filename, in char[] format)
+    {
+        return [tuple("", ""), tuple("0", "1"), tuple("Text in", "wird durch diesen ersetzt")];
+    }
+
+    loadSubstitutionFile!mockSlurpMultipleEntries("");
+    assert("" in map);
+    assert("0" in map);
+    assert("Text in" in map);
+    assert(map.length == 3);
+    assert(map[""] == "");
+    assert(map["0"] == "1");
+    assert(map["Text in"] == "wird durch diesen ersetzt");
+}
+
 auto substitute(string s) @safe nothrow
 {
     return s in map ? map[s] : s;