diff --git a/gssapi/raw/creds.pyx b/gssapi/raw/creds.pyx
index a4240751..d123857d 100644
--- a/gssapi/raw/creds.pyx
+++ b/gssapi/raw/creds.pyx
@@ -131,8 +131,11 @@ def acquire_cred(Name name=None, lifetime=None, mechs=None, usage='both'):
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t creds
     cdef gss_OID_set actual_mechs
@@ -227,8 +230,11 @@ accept_lifetime=None, mutate_input=False)
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:  # usage == 'both'
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t raw_input_cred
     if input_cred is not None:
diff --git a/gssapi/raw/ext_cred_store.pyx b/gssapi/raw/ext_cred_store.pyx
index 9ee57c36..ccfe0019 100644
--- a/gssapi/raw/ext_cred_store.pyx
+++ b/gssapi/raw/ext_cred_store.pyx
@@ -147,8 +147,11 @@ usage='both')
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_key_value_set_desc *c_store
     if store is not None:
@@ -232,8 +235,11 @@ init_lifetime=None, accept_lifetime=None)
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_name_t c_name = name.raw_name
     cdef gss_OID c_mech = &mech.raw_oid
@@ -325,8 +331,11 @@ set_default=False)
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_key_value_set_desc *c_store
     if store is not None:
diff --git a/gssapi/raw/ext_password.pyx b/gssapi/raw/ext_password.pyx
index be960f97..93f9c757 100644
--- a/gssapi/raw/ext_password.pyx
+++ b/gssapi/raw/ext_password.pyx
@@ -74,8 +74,11 @@ usage="initiate")
         c_usage = GSS_C_INITIATE
     elif usage == "accept":
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t creds
     cdef gss_OID_set actual_mechs
diff --git a/gssapi/raw/ext_password_add.pyx b/gssapi/raw/ext_password_add.pyx
index aed3cd05..c020063f 100644
--- a/gssapi/raw/ext_password_add.pyx
+++ b/gssapi/raw/ext_password_add.pyx
@@ -78,8 +78,11 @@ usage='initiate', init_lifetime=None, accept_lifetime=None)
         c_usage = GSS_C_INITIATE
     elif usage == "accept":
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef OM_uint32 input_initiator_ttl = c_py_ttl_to_c(init_lifetime)
     cdef OM_uint32 input_acceptor_ttl = c_py_ttl_to_c(accept_lifetime)
diff --git a/gssapi/raw/ext_rfc5588.pyx b/gssapi/raw/ext_rfc5588.pyx
index d87e6183..6244573b 100644
--- a/gssapi/raw/ext_rfc5588.pyx
+++ b/gssapi/raw/ext_rfc5588.pyx
@@ -63,8 +63,11 @@ set_default=False)
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t c_creds = creds.raw_creds
 
diff --git a/gssapi/raw/ext_s4u.pyx b/gssapi/raw/ext_s4u.pyx
index a1269b35..d91986e4 100644
--- a/gssapi/raw/ext_s4u.pyx
+++ b/gssapi/raw/ext_s4u.pyx
@@ -84,8 +84,11 @@ mechs=None, usage='initiate')
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t creds
     cdef gss_OID_set actual_mechs
@@ -162,8 +165,11 @@ usage='initiate', init_lifetime=None, accept_lifetime=None)
         c_usage = GSS_C_INITIATE
     elif usage == 'accept':
         c_usage = GSS_C_ACCEPT
-    else:
+    elif usage == 'both':
         c_usage = GSS_C_BOTH
+    else:
+        raise ValueError(f'Invalid usage "{usage}" - permitted values are '
+                         '"initiate", "accept", and "both"')
 
     cdef gss_cred_id_t raw_input_cred
     if input_cred is not None: