diff --git a/capa/features/common.py b/capa/features/common.py
index 782094d8..cf2c02f3 100644
--- a/capa/features/common.py
+++ b/capa/features/common.py
@@ -180,6 +180,7 @@ class String(Feature):
         super().__init__(value, description=description)
 
     def get_value_str(self) -> str:
+        assert isinstance(self.value, str)
         return escape_string(self.value)
 
 
@@ -236,6 +237,7 @@ class Substring(String):
             return Result(False, _MatchedSubstring(self, {}), [])
 
     def get_value_str(self) -> str:
+        assert isinstance(self.value, str)
         return escape_string(self.value)
 
     def __str__(self):
diff --git a/capa/features/extractors/common.py b/capa/features/extractors/common.py
index 585c0040..d72fcefd 100644
--- a/capa/features/extractors/common.py
+++ b/capa/features/extractors/common.py
@@ -9,6 +9,7 @@ import pefile
 import capa.features
 import capa.features.extractors.elf
 import capa.features.extractors.pefile
+import capa.features.extractors.strings
 from capa.features.common import OS, FORMAT_PE, FORMAT_ELF, OS_WINDOWS, FORMAT_FREEZE, Arch, Format, String, Feature
 from capa.features.freeze import is_freeze
 from capa.features.address import NO_ADDRESS, Address, FileOffsetAddress