diff --git a/CHANGES.txt b/CHANGES.txt
index 5b673a58..9b03a69e 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -36,6 +36,8 @@ Revision 4.4.5, released 2018-07-XX
 - Fixed possible infinite loop in GETBULK response PDU builder
 - Fixed memory leak in the `config.delContext()` VACM management harness
 - Fixed `Bits` class initialization when enumeration values are given
+- Fixed crash caused by incoming SNMPv3 message requesting SNMPv1/v2c
+  security model
 
 Revision 4.4.4, released 2018-01-03
 -----------------------------------
diff --git a/pysnmp/proto/mpmod/rfc2576.py b/pysnmp/proto/mpmod/rfc2576.py
index 93ded6da..717110d3 100644
--- a/pysnmp/proto/mpmod/rfc2576.py
+++ b/pysnmp/proto/mpmod/rfc2576.py
@@ -271,7 +271,7 @@ class SnmpV1MessageProcessingModel(AbstractMessageProcessingModel):
         try:
 
             try:
-                smHandler = snmpEngine.securityModels[int(securityModel)]
+                smHandler = snmpEngine.securityModels[securityModel]
 
             except KeyError:
                 raise error.StatusInformation(
diff --git a/pysnmp/proto/mpmod/rfc3412.py b/pysnmp/proto/mpmod/rfc3412.py
index 5f9268a5..58f3acc0 100644
--- a/pysnmp/proto/mpmod/rfc3412.py
+++ b/pysnmp/proto/mpmod/rfc3412.py
@@ -40,8 +40,11 @@ class HeaderData(univ.Sequence):
         namedtype.NamedType('msgMaxSize',
                             univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(484, 2147483647))),
         namedtype.NamedType('msgFlags', univ.OctetString().subtype(subtypeSpec=constraint.ValueSizeConstraint(1, 1))),
+        # NOTE (etingof): constrain SNMPv3 message to only USM+ security models
+        # because SNMPv1/v2c seems incompatible in pysnmp implementation, not sure
+        # if it's intended by the SNMP standard at all...
         namedtype.NamedType('msgSecurityModel',
-                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(1, 2147483647)))
+                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(3, 2147483647)))
     )