diff --git a/Changes b/Changes
index dc172e7..0e8520f 100644
--- a/Changes
+++ b/Changes
@@ -1,6 +1,8 @@
 0.43 2022-01-17
         - Fix temporary extract handling when defined
           in a config file.
+        - Improve handling of invalid certainty values
+          in TreeTagger.
 
 0.42 2022-01-11
         - Replaced Log4perl with Log::Any.
diff --git a/Makefile.PL b/Makefile.PL
index 6032c0d..ff1de01 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -45,6 +45,7 @@
     'Archive::Tar'    => 2.24,
     'Clone'           => 0.45,
     'List::Util'      => 1.45,
+    'Scalar::Util'    => 1.60,
   },
   MIN_PERL_VERSION => '5.016',
   test => {
diff --git a/lib/KorAP/XML/Annotation/TreeTagger/Morpho.pm b/lib/KorAP/XML/Annotation/TreeTagger/Morpho.pm
index 2e65abc..81fc525 100644
--- a/lib/KorAP/XML/Annotation/TreeTagger/Morpho.pm
+++ b/lib/KorAP/XML/Annotation/TreeTagger/Morpho.pm
@@ -1,6 +1,7 @@
 package KorAP::XML::Annotation::TreeTagger::Morpho;
 use KorAP::XML::Annotation::Base;
 use POSIX 'floor';
+use Scalar::Util 'looks_like_number';
 
 sub parse {
   my $self = shift;
@@ -28,7 +29,14 @@
         my $certainty = 0;
         foreach (@$content) {
           if ($_->{-name} eq 'certainty') {
-            $certainty = $_->{'#text'};
+
+            if (looks_like_number($_->{'#text'})) {
+              $certainty = $_->{'#text'};
+            }
+            else {
+              $certainty = 1;
+              $$self->log->warn('"' . $_->{'#text'} . '" is not a valid certainty value');
+            }
           }
           else {
             push @val, $_
diff --git a/t/annotation/corpus/doc/0001/tree_tagger/morpho.xml b/t/annotation/corpus/doc/0001/tree_tagger/morpho.xml
index 983ef05..c2c71bc 100644
--- a/t/annotation/corpus/doc/0001/tree_tagger/morpho.xml
+++ b/t/annotation/corpus/doc/0001/tree_tagger/morpho.xml
@@ -180,7 +180,7 @@
         <f name="lex">
           <fs>
             <f name="lemma">Betrieb</f>
-            <f name="certainty">1.000000</f>
+            <f name="certainty">'hurra'</f>
             <f name="ctag">NN</f>
           </fs>
         </f>
diff --git a/t/annotation/tt_morpho.t b/t/annotation/tt_morpho.t
index 616f08c..705f7d0 100644
--- a/t/annotation/tt_morpho.t
+++ b/t/annotation/tt_morpho.t
@@ -28,6 +28,8 @@
 is($data->{stream}->[10]->[3], 'tt/l:ein$<b>129<b>253', 'POS');
 is($data->{stream}->[10]->[4], 'tt/p:PTKVZ$<b>129<b>253', 'POS');
 
+is($data->{stream}->[13]->[3], 'tt/l:Betrieb', 'POS');
+
 is($data->{stream}->[-1]->[3], 'tt/l:werden', 'POS');
 is($data->{stream}->[-1]->[4], 'tt/p:VAFIN', 'POS');