diff --git a/stix2/v20/observables.py b/stix2/v20/observables.py
index 2b6c81ca..e079686c 100644
--- a/stix2/v20/observables.py
+++ b/stix2/v20/observables.py
@@ -230,9 +230,9 @@ class WindowsPEOptionalHeaderType(_STIXBase20):
         ('magic_hex', HexProperty()),
         ('major_linker_version', IntegerProperty()),
         ('minor_linker_version', IntegerProperty()),
-        ('size_of_code', IntegerProperty()),
-        ('size_of_initialized_data', IntegerProperty()),
-        ('size_of_uninitialized_data', IntegerProperty()),
+        ('size_of_code', IntegerProperty(min=0)),
+        ('size_of_initialized_data', IntegerProperty(min=0)),
+        ('size_of_uninitialized_data', IntegerProperty(min=0)),
         ('address_of_entry_point', IntegerProperty()),
         ('base_of_code', IntegerProperty()),
         ('base_of_data', IntegerProperty()),
@@ -246,15 +246,15 @@ class WindowsPEOptionalHeaderType(_STIXBase20):
         ('major_subsystem_version', IntegerProperty()),
         ('minor_subsystem_version', IntegerProperty()),
         ('win32_version_value_hex', HexProperty()),
-        ('size_of_image', IntegerProperty()),
-        ('size_of_headers', IntegerProperty()),
+        ('size_of_image', IntegerProperty(min=0)),
+        ('size_of_headers', IntegerProperty(min=0)),
         ('checksum_hex', HexProperty()),
         ('subsystem_hex', HexProperty()),
         ('dll_characteristics_hex', HexProperty()),
-        ('size_of_stack_reserve', IntegerProperty()),
-        ('size_of_stack_commit', IntegerProperty()),
-        ('size_of_heap_reserve', IntegerProperty()),
-        ('size_of_heap_commit', IntegerProperty()),
+        ('size_of_stack_reserve', IntegerProperty(min=0)),
+        ('size_of_stack_commit', IntegerProperty(min=0)),
+        ('size_of_heap_reserve', IntegerProperty(min=0)),
+        ('size_of_heap_commit', IntegerProperty(min=0)),
         ('loader_flags_hex', HexProperty()),
         ('number_of_rva_and_sizes', IntegerProperty()),
         ('hashes', HashesProperty(HASHING_ALGORITHM, spec_version="2.0")),
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index f4a4be0f..f35dcc03 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -297,8 +297,8 @@ class WindowsPEOptionalHeaderType(_STIXBase21):
         ('dll_characteristics_hex', HexProperty()),
         ('size_of_stack_reserve', IntegerProperty(min=0)),
         ('size_of_stack_commit', IntegerProperty(min=0)),
-        ('size_of_heap_reserve', IntegerProperty()),
-        ('size_of_heap_commit', IntegerProperty()),
+        ('size_of_heap_reserve', IntegerProperty(min=0)),
+        ('size_of_heap_commit', IntegerProperty(min=0)),
         ('loader_flags_hex', HexProperty()),
         ('number_of_rva_and_sizes', IntegerProperty()),
         ('hashes', HashesProperty(HASHING_ALGORITHM, spec_version="2.1")),