diff --git a/cal_tools/cal_tools/enums.py b/cal_tools/cal_tools/enums.py
index 186e17208300b400bd00640a0f3226369cbf9040..21d4b4f08eee3d8c95c2fbb09fe42dc177d1bff4 100644
--- a/cal_tools/cal_tools/enums.py
+++ b/cal_tools/cal_tools/enums.py
@@ -51,9 +51,9 @@ class SnowResolution(Enum):
 
 
 class AgipdGainMode(IntEnum):
-    """Encoding added to distinguish between adaptive and fixed gain"""
+    """Gain Modes where the values is 0 if Adaptive, or High/Medium/Low."""
 
-    ADAPTIVE_GAIN = 0  # adaptive is default (if gain mode missing in slow data)
-    FIXED_HIGH_GAIN = 1  # non-zero means fixed gain
+    ADAPTIVE_GAIN = 0
+    FIXED_HIGH_GAIN = 1
     FIXED_MEDIUM_GAIN = 2
     FIXED_LOW_GAIN = 3
diff --git a/cal_tools/cal_tools/tools.py b/cal_tools/cal_tools/tools.py
index a6aa3f5e8ee1ab02ae244a00e0daddd1d671d1dd..0ddf2c13264ee0eaddd8a26217b382376dcf80ef 100644
--- a/cal_tools/cal_tools/tools.py
+++ b/cal_tools/cal_tools/tools.py
@@ -675,6 +675,7 @@ def get_constant_from_db_and_time(karabo_id: str, karabo_da: str,
 
 def module_index_to_qm(index: int, total_modules: int = 16):
     """Maps module index (0-indexed) to quadrant + module string (1-indexed)"""
+    assert index < total_modules, f'{index} is greater than {total_modules}'
     modules_per_quad = total_modules // 4
     quad, mod = divmod(index, modules_per_quad)
     return f"Q{quad+1}M{mod+1}"
diff --git a/tests/test_cal_tools.py b/tests/test_cal_tools.py
index 7fe26e12a9fb96a9a38f006311726da13ae46559..c7656d1770a4bec28f935eae2c282306940b1b60 100644
--- a/tests/test_cal_tools.py
+++ b/tests/test_cal_tools.py
@@ -3,7 +3,7 @@ from pathlib import Path
 
 import pytest
 from cal_tools.plotting import show_processed_modules
-from cal_tools.tools import get_dir_creation_date
+from cal_tools.tools import get_dir_creation_date, module_index_to_qm
 
 
 def test_show_processed_modules():
@@ -30,3 +30,19 @@ def test_dir_creation_date():
     date = get_dir_creation_date(folder, 9999)
     assert isinstance(date, datetime)
     assert str(date) == '2019-12-16 08:52:25.196603'
+
+
+def test_module_index_to_qm():
+
+    assert module_index_to_qm(0) == 'Q1M1'
+    assert module_index_to_qm(1) == 'Q1M2'
+    assert module_index_to_qm(4) == 'Q2M1'
+    assert module_index_to_qm(6) == 'Q2M3'
+
+    assert module_index_to_qm(4, 5) == 'Q5M1'
+
+    with pytest.raises(AssertionError):
+        module_index_to_qm(18)
+
+    with pytest.raises(AssertionError):
+        module_index_to_qm(7, 5)