diff --git a/Sources/Plasma/CoreLib/plString.cpp b/Sources/Plasma/CoreLib/plString.cpp
index 754b7882..2b706612 100644
--- a/Sources/Plasma/CoreLib/plString.cpp
+++ b/Sources/Plasma/CoreLib/plString.cpp
@@ -171,18 +171,24 @@ void plString::IConvertFromUtf16(const uint16_t *utf16, size_t size)
             plUniChar unichar = 0x10000;
 
             if (sp + 1 >= utf16 + size) {
-                hsAssert(0, "Incomplete surrogate pair in UTF-16 data");
+                // Incomplete surrogate pair
                 unichar = BADCHAR_REPLACEMENT;
             } else if (*sp < 0xDC00) {
                 unichar += (*sp++ & 0x3FF) << 10;
-                hsAssert(*sp >= 0xDC00 && *sp <= 0xDFFF,
-                         "Invalid surrogate pair in UTF-16 data");
-                unichar += (*sp   & 0x3FF);
+                if (*sp < 0xDC00 || *sp > 0xDFFF) {
+                    // Invalid surrogate pair
+                    unichar = BADCHAR_REPLACEMENT;
+                } else {
+                    unichar += (*sp & 0x3FF);
+                }
             } else {
                 unichar += (*sp++ & 0x3FF);
-                hsAssert(*sp >= 0xD800 && *sp <  0xDC00,
-                         "Invalid surrogate pair in UTF-16 data");
-                unichar += (*sp   & 0x3FF) << 10;
+                if (*sp < 0xD800 || *sp >= 0xDC00) {
+                    // Invalid surrogate pair
+                    unichar = BADCHAR_REPLACEMENT;
+                } else {
+                    unichar += (*sp & 0x3FF) << 10;
+                }
             }
             *dp++ = 0xF0 | ((unichar >> 18) & 0x07);
             *dp++ = 0x80 | ((unichar >> 12) & 0x3F);
@@ -229,8 +235,8 @@ void plString::IConvertFromUtf32(const plUniChar *ustr, size_t size)
     const plUniChar *sp = ustr;
     while (sp < ustr + size) {
         if (*sp > 0x10FFFF) {
-            hsAssert(0, "UTF-32 character out of range");
-            convlen += 3;   // Use U+FFFD for release builds
+            // Invalid character gets replaced with U+FFFD
+            convlen += 3;
         }
         else if (*sp > 0xFFFF)
             convlen += 4;
diff --git a/Sources/Tests/CoreTests/test_plString.cpp b/Sources/Tests/CoreTests/test_plString.cpp
index efc1103d..9e3988b1 100644
--- a/Sources/Tests/CoreTests/test_plString.cpp
+++ b/Sources/Tests/CoreTests/test_plString.cpp
@@ -123,18 +123,30 @@ TEST(PlStringTest, ConvertInvalid)
 {
     // The following should encode replacement characters for invalid chars
     const plUniChar unicode_replacement[] = { 0xfffd, 0 };
+    const char latin1_replacement[] = "?";
 
-    const plUniChar char_too_big[] = { 0xffffff, 0 };
-    plUnicodeBuffer too_big = plString::FromUtf32(char_too_big).GetUnicodeArray();
+    // Character outside of Unicode specification range
+    const plUniChar too_big_c[] = { 0xffffff, 0 };
+    plUnicodeBuffer too_big = plString::FromUtf32(too_big_c).GetUnicodeArray();
     EXPECT_EQ(0, T_strcmp(unicode_replacement, too_big.GetData()));
 
-    // TODO: Invalid surrogate pairs can encode to 0xfffd, but it's handled
-    //       by an assert right now.
+    // Invalid surrogate pairs
+    const uint16_t incomplete_surr_c[] = { 0xd800, 0 };
+    plString incomplete_surr = plString::FromUtf16(incomplete_surr_c);
+    EXPECT_EQ(0, T_strcmp(unicode_replacement,
+                          incomplete_surr.GetUnicodeArray().GetData()));
+
+    const uint16_t double_low_c[] = { 0xd800, 0xd801, 0 };
+    plString double_low = plString::FromUtf16(double_low_c);
+    EXPECT_EQ(0, T_strcmp(unicode_replacement, double_low.GetUnicodeArray().GetData()));
+
+    const uint16_t bad_combo_c[] = { 0xdc00, 0x20, 0 };
+    plString bad_combo = plString::FromUtf16(double_low_c);
+    EXPECT_EQ(0, T_strcmp(unicode_replacement, bad_combo.GetUnicodeArray().GetData()));
 
     // ISO-8859-1 doesn't have \ufffd, so it uses '?' instead
-    const plUniChar high_char[] = { 0x1ff, 0 };
-    const char latin1_replacement[] = "?";
-    plStringBuffer<char> non_latin1 = plString::FromUtf32(high_char).ToIso8859_1();
+    const plUniChar non_latin1_c[] = { 0x1ff, 0 };
+    plStringBuffer<char> non_latin1 = plString::FromUtf32(non_latin1_c).ToIso8859_1();
     EXPECT_STREQ(latin1_replacement, non_latin1.GetData());
 }