diff --git a/gnu/local.mk b/gnu/local.mk
index 3332f10df8..d4d04c01b8 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -20,7 +20,7 @@
 # Copyright © 2018, 2019, 2020 Oleg Pykhalov <go.wigust@gmail.com>
 # Copyright © 2018 Stefan Stefanović <stefanx2ovic@gmail.com>
 # Copyright © 2018, 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
-# Copyright © 2019 Guillaume Le Vaillant <glv@posteo.net>
+# Copyright © 2019, 2020 Guillaume Le Vaillant <glv@posteo.net>
 # Copyright © 2019, 2020 John Soo <jsoo1@asu.edu>
 # Copyright © 2019 Jonathan Brielmaier <jonathan.brielmaier@web.de>
 # Copyright © 2019 Evan Straw <evan.straw99@gmail.com>
@@ -1572,6 +1572,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/rust-reproducible-builds.patch		 \
   %D%/packages/patches/rust-openssl-sys-no-vendor.patch	\
   %D%/packages/patches/rxvt-unicode-escape-sequences.patch	\
+  %D%/packages/patches/sbcl-geco-fix-organism-class.patch	\
   %D%/packages/patches/sbcl-graph-asdf-definitions.patch	\
   %D%/packages/patches/scalapack-blacs-mpi-deprecations.patch	\
   %D%/packages/patches/scheme48-tests.patch			\
diff --git a/gnu/packages/lisp-xyz.scm b/gnu/packages/lisp-xyz.scm
index 260fecb12d..fb85416e38 100644
--- a/gnu/packages/lisp-xyz.scm
+++ b/gnu/packages/lisp-xyz.scm
@@ -14186,7 +14186,8 @@ (define-public sbcl-geco
        (uri (string-append "https://common-lisp.net/project/geco/download/"
                            "geco-" version ".tar.gz"))
        (sha256
-        (base32 "0kk0bzr1019cfmf2b1jl1rk9shv3gx5z1znifxllg9mb98yqsgw0"))))
+        (base32 "0kk0bzr1019cfmf2b1jl1rk9shv3gx5z1znifxllg9mb98yqsgw0"))
+       (patches (search-patches "sbcl-geco-fix-organism-class.patch"))))
     (build-system asdf-build-system/sbcl)
     (home-page "https://common-lisp.net/project/geco/")
     (synopsis "Genetic algorithm toolkit for Common Lisp")
diff --git a/gnu/packages/patches/sbcl-geco-fix-organism-class.patch b/gnu/packages/patches/sbcl-geco-fix-organism-class.patch
new file mode 100644
index 0000000000..817596241e
--- /dev/null
+++ b/gnu/packages/patches/sbcl-geco-fix-organism-class.patch
@@ -0,0 +1,13 @@
+Fix the ORGANISM class so that SBCL >= 2.0.9 can compile it without error.
+
+--- a/classes.lisp	2020-10-28 12:11:10.725659464 +0100
++++ b/classes.lisp	2020-10-31 17:34:36.822752447 +0100
+@@ -148,7 +148,7 @@
+     :accessor score
+     :initarg :score
+     :initform 'nil
+-    :type number)
++    :type (or number null))
+    (NORMALIZED-SCORE
+     :accessor normalized-score
+     :initarg :normalized-score