diff --git a/moulinette/utils/filesystem.py b/moulinette/utils/filesystem.py
index f6e4f127..d400f103 100644
--- a/moulinette/utils/filesystem.py
+++ b/moulinette/utils/filesystem.py
@@ -22,7 +22,7 @@ def read_file(file_path):
     Keyword argument:
         file_path -- Path to the text file
     """
-    assert isinstance(file_path, basestring)
+    assert isinstance(file_path, basestring), "Error: file_path '%s' should be a string but is of type '%s' instead" % (file_path, type(file_path))
 
     # Check file exists
     if not os.path.isfile(file_path):
@@ -148,14 +148,14 @@ def write_to_file(file_path, data, file_mode="w"):
         file_mode -- Mode used when writing the file. Option meant to be used
         by append_to_file to avoid duplicating the code of this function.
     """
-    assert isinstance(data, basestring) or isinstance(data, list)
-    assert not os.path.isdir(file_path)
-    assert os.path.isdir(os.path.dirname(file_path))
+    assert isinstance(data, basestring) or isinstance(data, list), "Error: data '%s' should be either a string or a list but is of type '%s'" % (data, type(data))
+    assert not os.path.isdir(file_path), "Error: file_path '%s' point to a dir, it should be a file" % file_path
+    assert os.path.isdir(os.path.dirname(file_path)), "Error: the path ('%s') base dir ('%s') is not a dir" % (file_path, os.path.dirname(file_path))
 
     # If data is a list, check elements are strings and build a single string
     if not isinstance(data, basestring):
         for element in data:
-            assert isinstance(element, basestring)
+            assert isinstance(element, basestring), "Error: element '%s' should be a string but is of type '%s' instead" % (element, type(element))
         data = '\n'.join(data)
 
     try:
@@ -189,10 +189,10 @@ def write_to_json(file_path, data):
     """
 
     # Assumptions
-    assert isinstance(file_path, basestring)
-    assert isinstance(data, dict) or isinstance(data, list)
-    assert not os.path.isdir(file_path)
-    assert os.path.isdir(os.path.dirname(file_path))
+    assert isinstance(file_path, basestring), "Error: file_path '%s' should be a string but is of type '%s' instead" % (file_path, type(file_path))
+    assert isinstance(data, dict) or isinstance(data, list), "Error: data '%s' should be a dict or a list but is of type '%s' instead" % (data, type(data))
+    assert not os.path.isdir(file_path), "Error: file_path '%s' point to a dir, it should be a file" % file_path
+    assert os.path.isdir(os.path.dirname(file_path)), "Error: the path ('%s') base dir ('%s') is not a dir" % (file_path, os.path.dirname(file_path))
 
     # Write dict to file
     try: