diff --git a/src/simulation/Particle.cpp b/src/simulation/Particle.cpp
index 55a0b54d8..8963b9e75 100644
--- a/src/simulation/Particle.cpp
+++ b/src/simulation/Particle.cpp
@@ -33,11 +33,11 @@ std::vector<StructPropertyAlias> const &Particle::GetPropertyAliases()
 	return aliases;
 }
 
-std::vector<int> const &Particle::PossiblyCarriesType()
+std::vector<unsigned int> const &Particle::PossiblyCarriesType()
 {
 	struct DoOnce
 	{
-		std::vector<int> indices = {
+		std::vector<unsigned int> indices = {
 			FIELD_LIFE,
 			FIELD_CTYPE,
 			FIELD_TMP,
diff --git a/src/simulation/Particle.h b/src/simulation/Particle.h
index 4827c7122..3114093ba 100644
--- a/src/simulation/Particle.h
+++ b/src/simulation/Particle.h
@@ -18,7 +18,7 @@ struct Particle
 	 by higher-level processes referring to them by name such as Lua or the property tool **/
 	static std::vector<StructProperty> const &GetProperties();
 	static std::vector<StructPropertyAlias> const &GetPropertyAliases();
-	static std::vector<int> const &PossiblyCarriesType();
+	static std::vector<unsigned int> const &PossiblyCarriesType();
 };
 
 // important: these are indices into the vector returned by Particle::GetProperties, not indices into Particle