diff --git a/src/c_dispatch.cpp b/src/c_dispatch.cpp
index a826216fc..abd851b35 100644
--- a/src/c_dispatch.cpp
+++ b/src/c_dispatch.cpp
@@ -563,7 +563,7 @@ void C_DoCommand (const char *cmd, int keynum)
 	const char *beg;
 
 	// Skip any beginning whitespace
-	while (*cmd && *cmd <= ' ')
+	while (*cmd > 0 && *cmd <= ' ')
 		cmd++;
 
 	// Find end of the command name
@@ -575,7 +575,7 @@ void C_DoCommand (const char *cmd, int keynum)
 	else
 	{
 		beg = cmd;
-		for (end = cmd+1; *end > ' '; ++end)
+		for (end = cmd+1; *end > ' ' || *end < 0; ++end)
 			;
 	}
 
@@ -728,7 +728,7 @@ void AddCommandString (const char *text, int keynum)
 				more = 0;
 			}
 			// Intercept wait commands here. Note: wait must be lowercase
-			while (*cmd && *cmd <= ' ')
+			while (*cmd > 0 && *cmd <= ' ')
 				cmd++;
 			if (*cmd)
 			{
diff --git a/src/utility/zstring.cpp b/src/utility/zstring.cpp
index 51c3a1ffe..c3863387e 100644
--- a/src/utility/zstring.cpp
+++ b/src/utility/zstring.cpp
@@ -828,12 +828,12 @@ void FString::StripLeftRight ()
 	if (max == 0) return;
 	for (i = 0; i < max; ++i)
 	{
-		if (!isspace((unsigned char)Chars[i]))
+		if (Chars[i] < 0 || !isspace((unsigned char)Chars[i]))
 			break;
 	}
 	for (j = max - 1; j >= i; --j)
 	{
-		if (!isspace((unsigned char)Chars[j]))
+		if (Chars[i] < 0 || !isspace((unsigned char)Chars[j]))
 			break;
 	}
 	if (i == 0 && j == max - 1)