diff --git a/src/util/utf8.cpp b/src/util/utf8.cpp
index 1082ea25de..494b15f2f6 100644
--- a/src/util/utf8.cpp
+++ b/src/util/utf8.cpp
@@ -150,7 +150,7 @@ unsigned next_utf8(std::string const & str, size_t & i) {
     /* one continuation (128 to 2047) */
     if ((c & 0xe0) == 0xc0 && i + 1 < str.size()) {
         unsigned c1 = static_cast<unsigned char>(str[i+1]);
-        unsigned r = ((c & 0x1f) << 6) | c1;
+        unsigned r = ((c & 0x1f) << 6) | (c1 & 0x3f);
         if (r >= 128) {
             i += 2;
             return r;
@@ -161,7 +161,7 @@ unsigned next_utf8(std::string const & str, size_t & i) {
     if ((c & 0xf0) == 0xe0 && i + 2 < str.size()) {
         unsigned c1 = static_cast<unsigned char>(str[i+1]);
         unsigned c2 = static_cast<unsigned char>(str[i+2]);
-        unsigned r = ((c & 0x0f) << 12) | (c1 << 6) | c2;
+        unsigned r = ((c & 0x0f) << 12) | ((c1 & 0x3f) << 6) | (c2 & 0x3f);
         if (r >= 2048 && (r < 55296 || r > 57343)) {
             i += 3;
             return r;
@@ -173,7 +173,7 @@ unsigned next_utf8(std::string const & str, size_t & i) {
         unsigned c1 = static_cast<unsigned char>(str[i+1]);
         unsigned c2 = static_cast<unsigned char>(str[i+2]);
         unsigned c3 = static_cast<unsigned char>(str[i+3]);
-        unsigned r  = ((c & 0x07) << 18) | (c1 << 12) | (c2 << 6) | c3;
+        unsigned r  = ((c & 0x07) << 18) | ((c1 & 0x3f) << 12) | ((c2 & 0x3f) << 6) | (c3 & 0x3f);
         if (r >= 65536 && r <= 1114111) {
             i += 4;
             return r;
diff --git a/tests/lean/unicode_lit.lean b/tests/lean/unicode_lit.lean
new file mode 100644
index 0000000000..69438d5c55
--- /dev/null
+++ b/tests/lean/unicode_lit.lean
@@ -0,0 +1,2 @@
+#eval "≠∀Π∃⁻ʰℵ⬝"
+#eval "≠∀Π∃⁻ʰℵ⬝".to_list.map char.to_nat
diff --git a/tests/lean/unicode_lit.lean.expected.out b/tests/lean/unicode_lit.lean.expected.out
new file mode 100644
index 0000000000..2ab9ad6608
--- /dev/null
+++ b/tests/lean/unicode_lit.lean.expected.out
@@ -0,0 +1,2 @@
+"≠∀Π∃⁻ʰℵ⬝"
+[8800, 8704, 928, 8707, 8315, 688, 8501, 11037]