diff --git a/capa/features/extractors/viv/indirect_calls.py b/capa/features/extractors/viv/indirect_calls.py
index a090355a..8b04a3bc 100644
--- a/capa/features/extractors/viv/indirect_calls.py
+++ b/capa/features/extractors/viv/indirect_calls.py
@@ -7,7 +7,7 @@
 # See the License for the specific language governing permissions and limitations under the License.
 
 import collections
-from typing import TYPE_CHECKING, List, Tuple, Optional
+from typing import TYPE_CHECKING, Set, List, Deque, Tuple, Union, Optional
 
 import envi
 import vivisect.const
@@ -48,12 +48,14 @@ def get_previous_instructions(vw: VivWorkspace, va: int) -> List[int]:
     # ensure that it fallsthrough to this one.
     loc = vw.getPrevLocation(va, adjacent=True)
     if loc is not None:
-        # from vivisect.const:
-        # location: (L_VA, L_SIZE, L_LTYPE, L_TINFO)
-        (pva, _, ptype, pinfo) = vw.getPrevLocation(va, adjacent=True)
+        ploc = vw.getPrevLocation(va, adjacent=True)
+        if ploc is not None:
+            # from vivisect.const:
+            # location: (L_VA, L_SIZE, L_LTYPE, L_TINFO)
+            (pva, _, ptype, pinfo) = ploc
 
-        if ptype == LOC_OP and not (pinfo & IF_NOFALL):
-            ret.append(pva)
+            if ptype == LOC_OP and not (pinfo & IF_NOFALL):
+                ret.append(pva)
 
     # find any code refs, e.g. jmp, to this location.
     # ignore any calls.
@@ -72,7 +74,7 @@ class NotFoundError(Exception):
     pass
 
 
-def find_definition(vw: VivWorkspace, va: int, reg: int) -> Tuple[int, int]:
+def find_definition(vw: VivWorkspace, va: int, reg: int) -> Tuple[int, Union[int, None]]:
     """
     scan backwards from the given address looking for assignments to the given register.
     if a constant, return that value.
@@ -88,8 +90,8 @@ def find_definition(vw: VivWorkspace, va: int, reg: int) -> Tuple[int, int]:
     raises:
       NotFoundError: when the definition cannot be found.
     """
-    q = collections.deque()
-    seen = set([])
+    q = collections.deque()  # type: Deque[int]
+    seen = set([])  # type: Set[int]
 
     q.extend(get_previous_instructions(vw, va))
     while q: