diff --git a/lib/Parser/Legacy/NumberWithUnits.pm b/lib/Parser/Legacy/NumberWithUnits.pm
index e67859e8c..1a357ca14 100644
--- a/lib/Parser/Legacy/NumberWithUnits.pm
+++ b/lib/Parser/Legacy/NumberWithUnits.pm
@@ -92,7 +92,7 @@ sub splitUnits {
 		: $aUnit . '(?:\s*[/* ]\s*' . $aUnit . ')*';
 	$unitPattern = $unitPattern . '(?:\/' . $unitPattern . ')*' if $parseMathQuill;
 	my $unitSpace = "($aUnit) +($aUnit)";
-	my ($num, $units) = $string =~ m!^(.*?(?:[)}\]0-9a-z]|\d\.))\s*($unitPattern)\s*$!;
+	my ($num, $units) = $string =~ m!^(.*?(?:[)}\]0-9a-z]|\d\.))?\s*($unitPattern)\s*$!;
 	if ($units) {
 		while ($units =~ s/$unitSpace/$1*$2/) { }
 		$units =~ s/ //g;
@@ -238,20 +238,26 @@ sub unitsPreFilter {
 		$ans->{correct_value}{context}
 			&& $ans->{correct_value}->context->flag('useMathQuill')
 			&& (!defined $ans->{mathQuillOpts} || $ans->{mathQuillOpts} !~ /^\s*disabled\s*$/i));
+	if (defined($units) && $units ne '' && $num eq '') {
+		$self->cmp_Error($ans, "Units must follow a number");
+		$ans->{unit_error}  = $ans->{ans_message};
+		$ans->{student_ans} = '';
+		return $ans;
+	}
 	unless (defined($num) && defined($units) && $units ne '') {
 		$self->cmp_Error($ans, "Your answer doesn't look like " . lc($self->cmp_class));
-		$ans->{error_flag} = 'UNITS_NONE';
+		$ans->{unit_error} = $ans->{ans_message};
 		return $ans;
 	}
 	if ($units =~ m!/.*/!) {
 		$self->cmp_Error($ans, "Your units can only contain one division");
-		$ans->{error_flag} = 'UNITS_DIVISION';
+		$ans->{unit_error} = $ans->{ans_message};
 		return $ans;
 	}
 	my $ref = { getUnits($units) };
 	if ($ref->{ERROR}) {
 		$self->cmp_Error($ans, $ref->{ERROR});
-		$ans->{error_flag} = 'UNITS_BAD';
+		$ans->{unit_error} = $ans->{ans_message};
 		return $ans;
 	}
 	$ans->{units}       = $units;
@@ -263,6 +269,14 @@ sub cmp_preprocess {
 	my $self = shift;
 	my $ans  = shift;
 
+	if ($ans->{unit_error}) {
+		$ans->{ans_message} = $ans->{error_message} = $ans->{unit_error};
+		if ($ans->{student_ans} eq '') {
+			$ans->{student_ans}          = $ans->{original_student_ans};
+			$ans->{preview_latex_string} = TeXunits($ans->{student_ans});
+		}
+		return;
+	}
 	my $units = $ans->{units};
 	return $ans unless $units;
 	$ans->{student_ans}          .= " " . $units;
@@ -272,8 +286,8 @@ sub cmp_preprocess {
 	if (!defined($ans->{student_value}) || $self->checkStudentValue($ans->{student_value})) {
 		$ans->{student_value} = undef;
 		$ans->score(0);
-		$ans->{error_flag} = 'UNITS_NO_NUMBER';
 		$self->cmp_Error($ans, "Units must follow a number");
+		$ans->{unit_error} = $ans->{ans_message};
 		return;
 	}
 
@@ -284,7 +298,7 @@ sub cmp_preprocess {
 sub cmp_equal {
 	my $self = shift;
 	my $ans  = shift;
-	if (!$ans->{error_flag}) {
+	if (!$ans->{unit_error}) {
 		my $meth = @{ ref($self) . '::ISA' }[-1] . '::cmp_equal';
 		$meth = 'Value::cmp_equal' unless defined &$meth;
 		&$meth($self, $ans, @_);
@@ -298,7 +312,6 @@ sub cmp_postprocess {
 		$self->cmp_Error($ans, "The units for your answer are not correct")
 			unless $ans->{correct_value}->uPowers eq $ans->{student_value}->uPowers;
 	}
-	$ans->{error_flag} = undef if $ans->{error_flag} =~ m/^UNITS_/;
 	return $ans;
 }
 
@@ -350,7 +363,7 @@ sub makeValue {
 	my $value   = shift;
 	my %options = (context => $self->context, @_);
 	my $num     = Value::makeValue($value, %options);
-	return bless $num, 'Parser::Legacy::FormulaWithUnits' if $num->classMatch('Formula');
+	return bless $num, 'Parser::Legacy::FormulaWithUnits' if defined $num && $num->classMatch('Formula');
 	Value::Error("A number with units must be a constant, not %s", lc(Value::showClass($num)))
 		unless Value::isReal($num);
 	bless $num, $options{class};
diff --git a/macros/parsers/parserMultiAnswer.pl b/macros/parsers/parserMultiAnswer.pl
index 3472f64ab..2bc718521 100644
--- a/macros/parsers/parserMultiAnswer.pl
+++ b/macros/parsers/parserMultiAnswer.pl
@@ -104,8 +104,9 @@ sub cmp {
 	}
 
 	if ($self->{allowBlankAnswers}) {
+		my $blankCheck = AnswerEvaluator->new()->{pre_filters}[0][0];
 		foreach my $cmp (@{ $self->{cmp} }) {
-			$cmp->install_pre_filter('erase');
+			$cmp->{pre_filters} = [ grep { $_->[0] != $blankCheck } @{ $cmp->{pre_filters} } ];
 			$cmp->install_pre_filter(sub {
 				my $ans = shift;
 				$ans->{student_ans} =~ s/^\s+//g;
diff --git a/macros/parsers/parserRadioMultiAnswer.pl b/macros/parsers/parserRadioMultiAnswer.pl
index 8fdb61980..3cee48a56 100644
--- a/macros/parsers/parserRadioMultiAnswer.pl
+++ b/macros/parsers/parserRadioMultiAnswer.pl
@@ -354,9 +354,10 @@ sub cmp {
 	}
 
 	if ($self->{allowBlankAnswers}) {
+		my $blankCheck = AnswerEvaluator->new()->{pre_filters}[0][0];
 		for (@{ $self->{cmp} }) {
 			for my $cmp (@$_) {
-				$cmp->install_pre_filter('erase');
+				$cmp->{pre_filters} = [ grep { $_->[0] != $blankCheck } @{ $cmp->{pre_filters} } ];
 				$cmp->install_pre_filter(sub {
 					my $ans = shift;
 					$ans->{student_ans} =~ s/^\s+//g;
diff --git a/t/units/basic_parser.t b/t/units/basic_parser.t
index 6598c2981..9b96e6361 100644
--- a/t/units/basic_parser.t
+++ b/t/units/basic_parser.t
@@ -95,12 +95,8 @@ subtest 'Test error handling' => sub {
 		qr/Unrecognizable unit: \|$fake\|/,
 		"No unit '$fake' defined in Units file"
 	);
-	like(dies { NumberWithUnits(1) }, qr/You must provide units for your number/, 'No unit given');
-	like(
-		dies { NumberWithUnits('J') },
-		qr/You must provide units for your number/,
-		'No value given, wants 2 arguments'
-	);
+	like(dies { NumberWithUnits(1) },   qr/You must provide units for your number/,         'No unit given');
+	like(dies { NumberWithUnits('J') }, qr/A number with units must be a constant, not ''/, 'No value given');
 };
 
 subtest 'Check parsing of arguments' => sub {