diff --git a/clip.cpp b/clip.cpp
index b8afe72d..1b6afd4b 100644
--- a/clip.cpp
+++ b/clip.cpp
@@ -798,6 +798,13 @@ struct sorter {
 		mvt_value av = a.value(attr);
 		mvt_value bv = b.value(attr);
 
+        av = av.promote_for_comparison_with(bv);
+        bv = bv.promote_for_comparison_with(av);
+        if (av.type != bv.type) {
+            fprintf(stderr, "Sorter: can't happen\n");
+            exit(EXIT_IMPOSSIBLE);
+        }
+
 		return av < bv;
 	}
 };
diff --git a/mvt.cpp b/mvt.cpp
index 4c21716e..2511f744 100644
--- a/mvt.cpp
+++ b/mvt.cpp
@@ -470,6 +470,102 @@ bool mvt_value::operator<(const mvt_value &o) const {
 	return false;
 }
 
+mvt_value mvt_value::promote_for_comparison_with(mvt_value const &o) const {
+	// promotes this value either to string or double,
+	// depending on what it is intended to be compared with.
+	//
+	// comparison of a string and anything else is a string.
+	//
+	// (should there also be special cases for uint/sint/int
+	// for comparison of pairs of 64-bit integers? probably.)
+
+	if (o.type == mvt_string) {
+		if (type == mvt_string) {
+			return *this;
+		} else if (type == mvt_float) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = std::to_string(numeric_value.float_value);
+			return v;
+		} else if (type == mvt_double) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = std::to_string(numeric_value.double_value);
+			return v;
+		} else if (type == mvt_int) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = std::to_string(numeric_value.int_value);
+			return v;
+		} else if (type == mvt_uint) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = std::to_string(numeric_value.uint_value);
+			return v;
+		} else if (type == mvt_sint) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = std::to_string(numeric_value.sint_value);
+			return v;
+		} else if (type == mvt_bool) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = numeric_value.bool_value ? "true" : "false";
+			return v;
+		} else if (type == mvt_null) {
+			mvt_value v;
+			v.type = mvt_string;
+			v.string_value = "";
+			return v;
+		} else {
+			fprintf(stderr, "mvt_value::promote_for_comparison_with: can't happen\n");
+			exit(EXIT_IMPOSSIBLE);
+		}
+	} else {
+		if (type == mvt_string) {
+			return *this;
+		} else if (type == mvt_float) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.float_value;
+			return v;
+		} else if (type == mvt_double) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.double_value;
+			return v;
+		} else if (type == mvt_int) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.int_value;
+			return v;
+		} else if (type == mvt_uint) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.uint_value;
+			return v;
+		} else if (type == mvt_sint) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.sint_value;
+			return v;
+		} else if (type == mvt_bool) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = numeric_value.bool_value ? 1 : 0;
+			return v;
+		} else if (type == mvt_null) {
+			mvt_value v;
+			v.type = mvt_double;
+			v.numeric_value.double_value = 0;
+			return v;
+		} else {
+			fprintf(stderr, "mvt_value::promote_for_comparison_with: can't happen\n");
+			exit(EXIT_IMPOSSIBLE);
+		}
+	}
+}
+
 static std::string quote(std::string const &s) {
 	std::string buf;
 
diff --git a/mvt.hpp b/mvt.hpp
index 696e2535..f8d6b996 100644
--- a/mvt.hpp
+++ b/mvt.hpp
@@ -81,6 +81,7 @@ struct mvt_value {
 	} numeric_value;
 
 	bool operator<(const mvt_value &o) const;
+	mvt_value promote_for_comparison_with(mvt_value const &o) const;
 	std::string toString() const;
 
 	mvt_value() {