diff --git a/lib/abcsat/abc/satVec.h b/lib/abcsat/abc/satVec.h
index be404ff87..120328e43 100644
--- a/lib/abcsat/abc/satVec.h
+++ b/lib/abcsat/abc/satVec.h
@@ -130,7 +130,9 @@ static inline void   vecp_remove(vecp* v, void* e)
 typedef int    lit;
 typedef int    cla;
 
-typedef char               lbool;
+// Explicitly make it signed so promotion-to-int behavior doesn't vary
+// across platforms that define signedness of char differently.
+typedef signed char lbool;
 
 // CryptoMinisat defines it's own var_Undef values.
 // When it's included we prefer the ABC version instead.
diff --git a/lib/bill/bill/sat/solver/abc/satVec.h b/lib/bill/bill/sat/solver/abc/satVec.h
index 33b4eb489..121a64900 100644
--- a/lib/bill/bill/sat/solver/abc/satVec.h
+++ b/lib/bill/bill/sat/solver/abc/satVec.h
@@ -130,7 +130,9 @@ static inline void   vecp_remove(vecp* v, void* e)
 typedef int    lit;
 typedef int    cla;
 
-typedef char               lbool;
+// Explicitly make it signed so promotion-to-int behavior doesn't vary
+// across platforms that define signedness of char differently.
+typedef signed char lbool;
 
 // CryptoMinisat defines it's own var_Undef values.
 // When it's included we prefer the ABC version instead.