diff --git a/common/errors.c b/common/errors.c
index 05bb7415..88f54dfb 100644
--- a/common/errors.c
+++ b/common/errors.c
@@ -4,6 +4,26 @@
 #include "ui.h"
 #include "uipriv.h"
 
+#define internalErrorPrefix "libui internal error"
+// TODO add debugging advice?
+#define internalErrorSuffix "This likely means there is a bug in libui itself. Contact the libui authors."
+
+void uiprivInternalError(const char *fmt, ...)
+{
+	va_list ap;
+	char buf[256];
+	int n;
+
+	va_start(ap, fmt);
+	n = vsnprintf(buf, 256, fmt, ap);
+	va_end(ap);
+	if (n < 0)
+		uiprivReportError(internalErrorPrefix, "internal error string has encoding error", internalErrorSuffix, true);
+	if (n >= 256)
+		uiprivReportError(internalErrorPrefix, "internal error string too long", internalErrorSuffix, true);
+	uiprivReportError(internalErrorPrefix, buf, internalErrorSuffix, true);
+}
+
 static const char *messages[] = {
 	[uiprivProgrammerErrorWrongStructSize] = "wrong size %zu for %s",
 	[uiprivProgrammerErrorIndexOutOfRange] = "index %d out of range in %s()",
@@ -15,22 +35,19 @@ static void prepareProgrammerError(char *buf, int size, unsigned int which, va_l
 {
 	int n;
 
-	if (which >= uiprivNumProgrammerErrors) {
-		// TODO
-	}
+	if (which >= uiprivNumProgrammerErrors)
+		uiprivInternalError("bad programmer error value %u", which);
 	n = vsnprintf(buf, size, messages[which], ap);
-	if (n < 0) {
-		// TODO
-	}
-	if (n >= size) {
-		// TODO
-		buf[size - 4] = '.';
-		buf[size - 3] = '.';
-		buf[size - 2] = '.';
-		buf[size - 1] = '\0';
-	}
+	if (n < 0)
+		uiprivInternalError("programmer error string for %u has encoding error", which);
+	if (n >= size)
+		uiprivInternalError("programmer error string for %u too long (%d)", which, n);
 }
 
+#define programmerErrorPrefix "libui programmer error"
+// TODO add debugging advice?
+#define programmerErrorSuffix "This likely means you are using libui incorrectly. Check your source code and try again. If you have received this warning in error, contact the libui authors."
+
 void uiprivProgrammerError(unsigned int which, ...)
 {
 	va_list ap;
@@ -39,5 +56,5 @@ void uiprivProgrammerError(unsigned int which, ...)
 	va_start(ap, which);
 	prepareProgrammerError(buf, 256, which, ap);
 	va_end(ap);
-	uiprivSysProgrammerError(buf);
+	uiprivReportError(programmerErrorPrefix, buf, programmerErrorSuffix, false);
 }
diff --git a/common/uipriv.h b/common/uipriv.h
index 9fde1a53..4090117f 100644
--- a/common/uipriv.h
+++ b/common/uipriv.h
@@ -11,6 +11,7 @@ extern int uiprivInitReturnError(uiInitError *err, const char *msg);
 extern int uiprivInitReturnErrorf(uiInitError *err, const char *msg, ...);
 
 // errors.c
+extern void uiprivInternalError(const char *fmt, ...);
 enum {
 	uiprivProgrammerErrorWrongStructSize,		// arguments: size_t badSize, const char *structName
 	uiprivProgrammerErrorIndexOutOfRange,	// arguments: int badIndex, __func__
@@ -21,10 +22,7 @@ enum {
 	uiprivNumProgrammerErrors,
 };
 extern void uiprivProgrammerError(unsigned int which, ...);
-extern void uiprivSysProgrammerError(const char *msg);
-#define uiprivProgrammerErrorPrefix "libui programmer error"
-// TODO add debugging advice?
-#define uiprivProgrammerErrorAdvice "This likely means you are using libui incorrectly. Check your source code and try again. If you have received this warning in error, contact the libui authors."
+extern void uiprivReportError(const char *prefix, const char *msg, const char *suffix, bool internal);
 
 #ifdef __cplusplus
 }
diff --git a/darwin/main.m b/darwin/main.m
index db44cb60..05a3c063 100644
--- a/darwin/main.m
+++ b/darwin/main.m
@@ -115,12 +115,17 @@ void uiQueueMain(void (*f)(void *data), void *data)
 // - possibly others, all on stackoverflow.com (and maybe once on Apple's own forums?); I forget now
 static void debugBreak(void);
 
-void uiprivSysProgrammerError(const char *msg)
+void uiprivReportError(const char *prefix, const char *msg, const char *suffix, bool internal)
 {
-	NSLog(@"*** %s: %s. %s", uiprivProgrammerErrorPrefix, msg, uiprivProgrammerErrorAdvice);
-	// TODO either find an appropriate exception for each message or use a custom exception name
-	[NSException raise:NSInvalidArgumentException
-		format:@"%s: %s", uiprivProgrammerErrorPrefix, msg];
+	NSExceptionName exceptionName;
+
+	NSLog(@"*** %s: %s. %s", prefix, msg, suffix);
+	exceptionName = NSInternalInconsistencyException;
+	if (!internal)
+		// TODO either find an appropriate exception for each possible message or use a custom exception name
+		exceptionName = NSInvalidArgumentException;
+	[NSException raise:exceptionName
+		format:@"%s: %s", prefix, msg];
 	debugBreak();
 	abort();		// we shouldn't reach here
 }