diff --git a/pyerrors/input/sfcf.py b/pyerrors/input/sfcf.py
index c91c8fcb..94aa1948 100644
--- a/pyerrors/input/sfcf.py
+++ b/pyerrors/input/sfcf.py
@@ -237,8 +237,14 @@ def read_sfcf_multi(path, prefix, name_list, quarks_list=['.*'], corr_type_list=
             if "files" in kwargs:
                 files = kwargs.get("files")
                 if isinstance(files, list):
-                    if isinstance(files[0], list):
+                    if all(isinstance(f, list) for f in files):
                         files = files[i]
+                    elif all(isinstance(f, str) for f in files):
+                        files = files
+                    else:
+                        raise TypeError("files has to be of type list[list[str]] or list[str]!")
+                else:
+                    raise TypeError("files has to be of type list[list[str]] or list[str]!")
 
             else:
                 files = []
@@ -313,7 +319,10 @@ def read_sfcf_multi(path, prefix, name_list, quarks_list=['.*'], corr_type_list=
             w = specs[3]
             w2 = specs[4]
             if "files" in kwargs:
-                name_ls = kwargs.get("files")
+                if isinstance(kwargs.get("files"), list) and all(isinstance(f, str) for f in kwargs.get("files")):
+                    name_ls = kwargs.get("files")
+                else:
+                    raise TypeError("In append mode, files has to be of type list[str]!")
             else:
                 name_ls = ls
                 for exc in name_ls:
diff --git a/tests/sfcf_in_test.py b/tests/sfcf_in_test.py
index 5759ccf9..f92126f9 100644
--- a/tests/sfcf_in_test.py
+++ b/tests/sfcf_in_test.py
@@ -144,6 +144,13 @@ def test_c_bi_files(tmp_path):
     assert f_A[2].value == -41.025094911185185
 
 
+def test_c_bi_files_int_fail(tmp_path):
+    build_test_environment(str(tmp_path), "c", 10, 3)
+    with pytest.raises(TypeError):
+        sfin.read_sfcf(str(tmp_path) + "/data_c", "data_c", "f_A", quarks="lquark lquark", wf=0, version="2.0c",
+                       files=[[range(1, 11, 2)], [range(2, 11, 2)], [range(1, 11, 2)]])
+
+
 def test_c_bib(tmp_path):
     build_test_environment(str(tmp_path), "c", 5, 3)
     f_V0 = sfin.read_sfcf(str(tmp_path) + "/data_c", "data_c", "F_V0", quarks="lquark lquark", wf=0, wf2=0, version="2.0c", corr_type="bib")
@@ -291,6 +298,13 @@ def test_a_bi_files(tmp_path):
     assert f_A[2].value == -41.025094911185185
 
 
+def test_a_bi_files_int_fail(tmp_path):
+    build_test_environment(str(tmp_path), "a", 10, 3)
+    with pytest.raises(TypeError):
+        sfin.read_sfcf(str(tmp_path) + "/data_a", "data_a", "f_A", quarks="lquark lquark", wf=0, version="2.0a",
+                       files=[[range(1, 11, 2)], [range(2, 11, 2)], [range(1, 11, 2)]])
+
+
 def test_a_bib(tmp_path):
     build_test_environment(str(tmp_path), "a", 5, 3)
     f_V0 = sfin.read_sfcf(str(tmp_path) + "/data_a", "data_a", "F_V0", quarks="lquark lquark", wf=0, wf2=0, version="2.0a", corr_type="bib")