1 files changed, 10 insertions, 1 deletions
diff --git a/quality_control/headers.py b/quality_control/headers.py
index b8ff160..59d3d77 100644
--- a/quality_control/headers.py
+++ b/quality_control/headers.py
@@ -4,4 +4,13 @@ from quality_control.errors import InvalidHeaderValue
 
 def valid_header(strains, headers):
     "Return the valid headers with reference to strains or throw an error"
-    return None
+    if not bool(headers[1:]):
+        raise InvalidHeaderValue(
+        "The header MUST contain at least 2 columns")
+    invalid_headers = tuple(
+        header for header in headers[1:] if header not in strains)
+    if invalid_headers:
+        raise InvalidHeaderValue(
+            *(f"'{header}' not a valid strain." for header in invalid_headers))
+
+    return headers