diff --git a/passes/techmap/libparse.cc b/passes/techmap/libparse.cc
index bc7758cbd..97a94ece0 100644
--- a/passes/techmap/libparse.cc
+++ b/passes/techmap/libparse.cc
@@ -75,7 +75,7 @@ bool LibertyInputStream::extend_buffer_once()
 		buffer.resize(buf_end + chunk_size);
 	}
 
-	size_t read_size = f.rdbuf()->sgetn(buffer.data() + buf_end, chunk_size);
+	size_t read_size = f.rdbuf()->sgetn((char *)buffer.data() + buf_end, chunk_size);
 	buf_end += read_size;
 	if (read_size < chunk_size)
 		eof = true;
diff --git a/passes/techmap/libparse.h b/passes/techmap/libparse.h
index 3ba381fdf..f834044ae 100644
--- a/passes/techmap/libparse.h
+++ b/passes/techmap/libparse.h
@@ -92,7 +92,7 @@ namespace Yosys
 
 	class LibertyInputStream {
 		std::istream &f;
-		std::vector<char> buffer;
+		std::vector<unsigned char> buffer;
 		size_t buf_pos = 0;
 		size_t buf_end = 0;
 		bool eof = false;
@@ -107,7 +107,7 @@ namespace Yosys
 		LibertyInputStream(std::istream &f) : f(f) {}
 
 		size_t buffered_size() { return buf_end - buf_pos; }
-		const char *buffered_data() { return buffer.data() + buf_pos; }
+		const unsigned char *buffered_data() { return buffer.data() + buf_pos; }
 
 		int get() {
 			if (buf_pos == buf_end)